I have a 1966 Cutlass. Inside the webbing in the back seat I found a card. It's index card size but paper. Long list of 3 digit codes. Looks to be hand written in pencil! I left it where is as is. Nobody I have talked to (including myself) has ever seen one like this before but it has to be a build sheet.
I'm thinking of selling the car so I think it may be worth something as I have absolutely everything this car would have come with (including every receipt for maintenance) and it's still 99% original. I bought it from the original owner and parked it the last 15 years. The old lady I bought it didn't drive it much (84k UN-documented) and it's a good thing she didn't. She must have ran into everything she drove by.
Any info would be greatly appreciated (still never seen hand written in pencil).

 

Please post a scan/pic. It is most likely a Fisher Body Broadcast Card. I have seen a couple examples of hand written ones, my guess is your car is a Fremont CA car?