I found a build sheet behind the seat in my car but its not original to my car as the vin# doesnt match so the seat must be out of a differnt car if anyone is looking for a build sheet for vin # 1Q87L8L533089 the car was built in van nuys,ca but ordered to wetaskwin motors limited in wetaskwin alberta canada

It is definitely possible that the seat could have been changed out over the years, but many times the build sheets ended up in the wrong cars on the assembly line.

If the build sheet is for the same year car and was built around the same time as yours, you can almost bet that it was put there on the assembly line.

I found one 1970 Camaro in the wrecking yard with a1970 Monte Carlo build sheet in the rear seat. They were both built the same day in Van Nuys.

If your car is a different year, or comes from a different assembly plant, or was made at a different time, that would indicate that the seat has been changed.

This is an interesting subject. I was researching my car, which the original owner says he ordered to Edmonton. It is an LA built car. When I called GM Canada to see what paperwork I could get on it, they told me that LA built Camaro's were never sent to Canada. They were pretty blunt with it and told me I was out of luck.
So that build sheet you have is pretty interesting material I think.
By the way, I found a 70 Monte Carlo build sheet behind my rear seat. I found my real one under the front right seat later on. It isn't in good shape. Im going in search of the one on top of the gas tank soon.