RPO (Regular Production Option Number) - Interior / Exterior Trim packages Chevy offered: 

Custom - (base model) - Black rubber gasket holding the windshield in. White painted bumpers and mirrors. Vent window pillars and frames were flat black. Standard painted hub caps. Tail light and tailgate moldings were NOT included or offered.

Custom Deluxe - Full width comfortably padded bench seat and padded arm rests on the door panels. Padded flip down sun visors. Vinyl seat upholstery and matching vinyl on the door panels. The interior roof panel was painted to match the exterior color of the truck. A black rubber floor mat that went from the edge of the front seat up to the firewall. Courtesy lamps, and a padded dashboard. Stainless steel upper and lower grille moldings. Chrome headlight bezels. Stainless steel mirrors. White painted front bumper. Chromed Custom Deluxe emblems on the fenders.

 Z84 Cheyenne - Bench seat with full depth padded seat and back rest, with all vinyl or vinyl and nylon cloth seat covering. Door trim panels and headliner. Nylon carpet from front edge of seat to firewall. Color keyed moldings. Ash tray mounted cigar lighter. Custom steering wheel. Cheyenne dashboard emblem. Door pin switch or manually operated courtesy lights. Extra insulation to block noise. Upper body moldings. Tailgate trim panel with Cheyenne emblem. Blazers that were ordered with the Cheyenne package, had a chrome round emblem on the rear fenders.

Cheyenne Super - This added upper and lower body side moldings, a chrome tailgate handle, chrome wheel well moldings, and Cheyenne Super emblems on the dash and fenders. 

Silverado - (See 1975 Model Year)

Scottsdale - (See 1975 Model Year)

Optional Equipment  RPO (Regular Production Options)
Chrome front  bumper, chrome rear bumper, rear step bumper, AM radio, AM/FM radio, clock, power steering, below eye line mirrors, cab drip moldings, sliding rear window, cargo lamp, gauge package, A/C, tachometer, tilt steering wheel, exterior tool storage compartment, wheel covers, white wall tires, exterior wood grain trim, and a glide out under body spare tire carrier. There were also platform (flat bed), and stake (rack) body optional diesel, and LPG (Low Pressure Gas) engines. For the Blazer, the rear roof was actually an option! This is why they are referred to as "Open Utility".

1973 Production Data By Model

1974 - Full-time 4wd was offered in V-8 models, using the NP 203 fulltime 4wd transfer case. The 307 V-8 was discontinued, 6cyl engines were not available in California due to emissions standards. The grille had an emblem with the engine size on it. Same trim packages available as 1973. The NP 203 was still used in some manual transmission equipped trucks.

1975 - There were quite a few changes this year. The 400 V-8 was offered in trucks over 6,000 pounds GVWR. All pick ups below 6,000 pounds were catalytic converter equipped. All V-8 trucks with auto transmissions used the NP 203 fulltime 4wd transfer case, all manual trans trucks used the NP 205 transfer case, and manual locking hubs. The grille still had the engine size badge. Trim packages offered: Custom Deluxe was the base model, replacing Custom. The next level was Scottsdale. The new top of the line Silverado model was introduced.

Z62 Scottsdale - Came with all the items in the Custom Deluxe model, plus bright trim on bench seats, door or manually operated dome light, full length bright trim door sill plates, high note horn, and more. Front seat folds forward for storage. The exterior features fully chromed bumpers, in addition to all the items in the Custom Deluxe model. Pus bright hub caps, bright windshield trim, bright rear window trim, bright-trimmed marker lamps and tail lamps, and Scottsdale name plates.

YE9 Silverado - This came with all the goodies included in the Cheyenne package, and full gauge instrumentation, instead of just warning lights. Full door trim panels with matching carpet on the bottom sections, door pockets, full cowl trim in the floor area, and Silverado name plate. The exterior includes full upper and lower body side moldings, wheel opening moldings, full center tailgate panel, and a bright trim panel across the back of the cab just above the top of the bed.

1976 - The NP 203 fulltime 4wd transfer case was used with all automatic trans equipped trucks. All manual transmission equipped trucks used the NP 205 t case. This was the first year of the new roof design on the Blazer. Different from earlier years, the rear fiberglass roof detached from the cab behind the driver. This was done for safety sake. Prior years, the roof detached from the windshield frame, making it a complete convertible. The engine size emblem on the grille was discontinued. Trim packages were the same, though there were special options offered every year. Including some type of Bicentennial model. The Blazer Chalet debuted this year.

Here is an example of a 76 2wd

1977 - The one ton K/30 model was introduced, along with the Bonus cab, and Crew Cab (3+3) models. The bonus cab was the same as a crew cab, minus the rear seat. (Chevy did not offer any extended cab models like today's during 73 - 87). The trim on the Silverado featured a 'mustard' colored strip down the center. This is the only model year to feature this color stripe. Grille design was changed. Power door locks were offered, and late in the model year, power windows were offered also. K/20 models were available with GVWR from 6,800 - 8,400 pounds, as in earlier years. Something was changed with the doors, because 77 - 87 doors are the same part#. Trim packages offered: Custom Deluxe, Scottsdale, Cheyenne, and Silverado. The BIG 10 was also available this year (I am unclear if it was available earlier). The Big 10 was a heavy half ton, with GVWR of 6,050 (6 and V-8 engines) and 6,200 (V-8 only). The "Big Dooley" was basically a dual rear wheel truck, with the Camper Special package, and Trailering Special package.

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From a 1977 GM sales brochure:

"Trailering Special Packages - Recommended for trailer towing, Chevy Trailering Special Packages are available for both 2wd and 4wd vehicles in all series. The recommended equipment will vary depending on the series and the weight to be towed; light, medium, or heavy. Equipment specified in most includes a 350 - 4 bbl, or a 454 4bbl V-8 engine, Turbo Hydra-matic (TH) transmission, power steering, power brakes, HD battery, trailer wiring harness, weight distributing hitch, or ball hitch for step bumper."

"Camper Special Packages - Available on all 20 and 30 series with V-8 and turbo hydra-matic transmission or 4 speed manual transmission, Camper Special Packages are recommended for use with slide in camper bodies for pickups or chassis mounted bodies for cab and chassis models. Although equipment recommendations will vary with body weight, most packages specify camper body wiring harness, heavy duty front and rear springs, heavy duty front and rear shock absorbers, heavy duty front stabilizer bar, and Camper Special name plate. Deluxe Camper Special Package includes all the equipment in the Camper Special Package, plus these additions: Camper Tie Down Package: Elimi-Pitch Package: rear stabilizer bar (standard on dual rear wheel vehicles). Available on Fleetside models only.

Plus..... Power Steering, power brakes, electric clock, chromed front and rear bumpers, bumper guards, two-tone paint, fuel tank shield plate(s), engine oil cooler, 61-amp generator, heavy duty shocks absorbers, heavy duty front and rear springs, heavy duty battery, color keyed front floor mats, intermittent windshield wipers, Rally wheels, styled steel wheels, inside hood lock release, weight distributing trailer hitch platform, ball type trainer hitch and a wide array of factory installed tires."

SERIES	PAYLOAD #	Here is a 77 Blazer from Paris, France
C10	960 - 2680
BIG - 10	1980 - 2355
C20	2075 - 3930
C30	2125 - 5720
K10	1790 - 2495
K20	2120 - 4250
K30	3795 - 5345

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1978 - Frame changes were made to fit catalytic converters. Trucks with V-8's and 4wd, had struts added between the engine and transmission, and other struts from the trans to t case. This was the last year the Dana 44 front axle was used (though it does appear in later models). This is the first year power windows were offered. Late in the model year, a fuel door was added, leaving no fuel cap exposed on the bedside. This began with models using unleaded fuel. Trim packages offered: same as last year.

Here is an example of a 78 Chevy

1979 - No major changes other than trim and paint schemes. The headlight bezels incorporated the front turn signals, unlike earlier models. Some think this is the best looking grille and bezel set up. Actually very similar looking to Dodge's. Same trim packages.

1980 - The V-8 engines now have the dipstick on the passenger side. The NP 203 fulltime 4wd transfer case was no longer used. All V-8 4wd's had the NP 205 transfer case. The 400 small block was no longer available in K/10 models. The headlight bezels are the same as 1979, but the grille has more chrome across the bottom. Silverado's used rectangular headlamps. The K/20's maximum GVWR was increased to 8,600 pounds, to avoid the use of catalytic converters. Trim packages offered were the same as earlier years.

1981 - The front end sheet metal was redesigned. This consisted of redesigned fenders, with the side marker lights horizontal instead of vertical. The hood, became a 2 piece design, with the cowl separate from the hood. This also required the hood hinges to be changed from earlier years. They went from an extension type spring, to a coiled flat spring. The hinge recessed into the cowl area. Earlier model years, the hinges were just bolted to the inside of the fenders. The cab and doors remained the same, with only the interior panels and vent windows changing. The interior was redesigned, including the seats and dash panels. The NP 208 aluminum transfer case was now used in K/10 and K/20 models. All K series trucks had manual locking hubs. The 400 small block was no longer offered. (special order I'm sure though). The 454 big block was made available in K models. The fuel tank was moved to the drivers side frame rail, still on the outside of the frame. The 305 V-8 was now offered in the K series. There were now 8 different engines offered. Trim packages remained the same.

1982 - The 6.2L 379 diesel was offered. It came with a special 4 speed overdrive manual transmission. The automatic TH 350 transmission, was replaced with the TH700-R4 with overdrive in all K/10 and K/20 models, and K/10 diesels. T case gears in K/30 trucks were now synchronized, allowing shifting into 4 Hi below 25 mph. The Cheyenne trim package was discontinued, with the others still remaining. Custom Deluxe, Scottsdale, and Silverado. This year featured the front marker light / turn signals mounted in the front bumper.

Here is an example of an 82

1983 - No notable changes, other than the front marker lights being moved back to the front grille instead of the front bumper.  

1984 - A few trim changes on the interior and exterior. the manual 4 speed was offered in models other than diesels.  

1985 - The 250 inline 6 was replaced by the 4.3L V-6. The grille design changed. Trim packages remained the same.

Here is an example of an 85

1986 - No notable changes were made.

Here is an example of an 86 2wd Silverado

1987 - The last year of this body and chassis style for the pick up line. The Suburban and Blazers used the same design until 1991. The 4wd designation of "K" was replaced with "V". All engines were now fuel injected via Throttle Body Injection (TBI). The ignition system was now computer controlled. The same bed and tail lights were used for the entire production run from 1973 to 1987. They all bolt right onto each other and can be swapped between years.
 

Here is an example of an 87

Here is an example of a 1991 Blazer

There were a few promotional limited production trim packages. GM was always buyer conscious, and really wanted to cater to different types of customers. GMC had special packages and model offerings over the years too.
One group they targeted was young men. The models aimed at them were the "Chevy Sport", and GMC's "Beau James".
The Chevy Sport was available in step side and fleetside models. The Sport featured wide, multi-tone decal striping, rally wheels, and white lettering.
The Beau James had a silver and blue paint scheme and "Gone With The Wind" style Beau James decals on the rear of the pickup box. Also a Beau James hood ornament, front bumper guards, driving lights, and chrome bed rails.

You can see a 1975 Beau James here.

Other promotional packages offered were the Chevy "Bonanza", the "Outdoorsman", and the GMC Jimmy "Gentleman Jim".
The Bonanza had Bonanza emblems on the front fenders. It was a sales incentive program of some sort.
The Outdoorsman was also a sales incentive program that offered buyers hunting, fishing, and camping equipment, when they bought a Chevy truck. Outdoorsman trucks had the Outdoorsman decal on the rear of the bed on each side.
The GMC Gentleman Jim was black and silver, had gold interior and exterior trim, a gold grille, and bucket seats.
Another very rare model was the 1976 - 77 Balzer Chalet. The GMC model equivalent offered was the 1976 "Jimmy Casa Grande". This was a GMC Jimmy, with a camper cap over the back. It had a pop up top that gave 6'7" of head room. The cab allowed easy access to the camper. The camper was designed to sleep two. There were optional overhead bunks to sleep four. The camper was equipped with an LP gas stove, and a refrigerator too.
Somewhere between 83 and 85, there was a Sno Chaser model offered.
The Sno Chaser had a cab spoiler mounted on the bed behind the cab, and decals on the rear bedsides saying "Sno Chaser".
These were all unique models that collectors should look for. They all had very low production numbers, some lower than others.

Factory Service Manuals!

GM Light Duty Truck Service Manuals cover the following models: C/10, C/20, C/30, K/10, K/20, K/30, P/10, P/20, P/30. (C= 2wd pickups, cab n' chassis, and Blazer / Suburban, K= 4wd pickups, cab n' chassis, and Blazer / Suburban, and the P model line is Chevy Vans. GM considers anything 1 ton or under to be a "Light Duty" vehicle.

So if you plan on restoring a car or truck, get your hands on a factory service manual, you will not be disappointed!