I suppose you have no doubt double checked the seats.
I could be wrong about earlier than 78, but at least in 78 there was a choice of seats.
The Pace Car & the Anniversary finished car had optional seats that folded higher up, were fuller, with seat and back bolstering.
These were then the base seats for the following 79 thru 82 models.
My basic 78 seats are flatter, and do NOT have a slot high up across the back swab.
I've seen earier cars with this type of seat, the Pace car and later 79~82 seats are more comfortable, and tend to hold you in place, whilst on the original seats, I used to slide from side to side. (No wonder I like C4 seats)