Found a second buildsheet in my car??

So earlier today as i was cleaning the back seat area( deck and seat) for glass, and to get certain things painted. While i was there i decided to remove the seat to prevent overspray. Now This was the very first time i had removed the bottom part of the rear seat. I found another buildsheet and this one is in better condition than the first i found. I honestly dont even think the first was to my car.. The first buildsheet was partly burned, ripped, and said different things which is why im pretty sure it wasnt to this car.. But they were both for 72 darts, so why would this be, do you think someone swapped in a different seat at once? Now the last buildsheet is lost(i think?) i know i have it somewhere but theres so much crap everywhere i would have to look very hard and its only about a quarter size of the new one i found :( does anyone know what all the codes mean? In particular the axle codes and what "H series 1972" means at the top of the sheet? I know i write alot lol but i would like some help. Thanks