I have found a build sheet under carpet.
Neither of these were from Hamtramck but that doesn't mean that it doesn't happen.
I have heard of one person who found one between the gas tank and the vibration mat.
Ya never know.
Is your car fe5 red also? It must have an E4 interior since this build sheet came from the seat.
Often times the seat was put together and stacked with several other seats that are exactly the same.
When the line workers would grab the seat they would just put the correct looking seat in the car and not worry about the actual vin of a car just as long as the seat was correct for the car.
Now I was not there and do not claim to know any of this as fact as I wasn't even alive. I'm only going off of what I have read and what the old timers have stated on numerous threads and in conversations.
That build sheet does indeed help you, in a round about way, prove your car was indeed a red interior car. Same SPD. Or at least very similar as your mylar states September and the build sheet also states september. Pretty cool provinance to your car. I would photo copy that sheet several times on proper paper and keep several copies with your car regardless of what you do with the BS.
If it gets sent to the registry it will be unavailable to be seen or properly photo scanned ever again.
Copy, Copy, Copy, Copy. And post on as many sites as you can.
Try posting it over on ebodies.org also.