The vin sites claim to accurate. But the reality is how much effort they want to put into the actual gathering of existing information. We have big advantage over other manufacturers as GM tells you right up front, on every vehicle equipped with a Service Parts Identification Tag, Exact Model Code, and all options it was built with. In the service manual for your year truck, suv, etc, all the three digit option codes available for that year are listed. So there won't be code for a convertible, or a 500 cubic inch. Only what you could get that year. It narrows down the list if your wanting to decode what your truck has.
The first twenty pages of the service manual explain the " birth " of your truck what it is, what it's got, and the three digit code it speaks.
As Mr. Shurkey, calls it, at the "treasure yard" , don't know exactly what? Drop the glove box door check the codes, get treasure!
Your using the service manual, vehicle information section will give you more information than any pay vin or free vin website. And the manuals are free here to download. What a deal! Good luck!