Styling. The studio thought it looked unfinished with the trim on the door and cab, and then nothing on the box.
Options are not coded in the VIN. For some model years you get the engine option, for warranty tracking purposes, but color/trans/axle/anything else is not coded in the VIN.
Build sheets are not placed anywhere intentionally but were used to facilitate the build only. They were intended to be cleaned out and thrown away at the end of the final line just like extra screws, coffee cups, newspapers, etc. Sometimes it was easier to leave them in place then to remove them which is why you often find them in the seat springs, behind the headliner, in the A pillars, under the carpet or on top of the fuel tank (for passenger cars). If you find one in a vehicle you should consider yourself very very lucky, because someone was not doing their job.
If your vehicle was US built and after the 1976 (GMC) or 1977 model year (Chevy) you can send $50 and your VIN to the GM Heritage Center archive and they will return to a copy of the original invoice, which includes RPO content, pricing, original selling dealer information and build dates.
http://www.gmmediaarchive.com/?page=1
If your vehicle was built in Canada or built in the US for Canadian use then GM of Canada would have the option content by VIN.
https://www.vintagevehicleservices.com/option-info
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