[unruhjonny]

the rear seats can be grouped into three types - these are all my personal observations:

1970-1973: thick; spring and burlap/cotton backing behind covers
1974-1976(?); thick; frame with formed foam backing
1977(?)-1981; thin; frame with formed foam backing

I mistakenly tossed my car's original rear seatback shortly after getting it, because I had a nice '77 velour seat in the rear;
I ordered replacement seat covers in 2001(?), and quickly found out that the replacement seat back cover was too big. The replacement covers were obviously for a "thicker" seat back - and I was happy at that time that I kept both original seat bottoms - I ended up using a donor '73 seatback (from my former '73 Firebird; and it took a while to find another one for that car).

It would be quite possible that the original burlap/cotton material was replaced by foam at some point after being built, but from my personal observations formed foam padding replaced the cotton/burlap material after the 1973 model year.
I am not certain what was used for 1974, but I have observed that 1975 used the thick style seat back with a formed foam pad ontop of the seat frame.

About the "different plants used different foams";
For 1970 & 1971 there were only two plants that built the F-body;
Norwood Ohio & Van Nuys California.
For 1972 & 1973 only Norwood built them.

I believe that the 1971-1972 (front) seat foams continued to be used on the 1973-1977 'standard' seats - while the 1973-1977 deluxe seats used the "horseshoe" style seats which were to my understanding a version of the Grand Prix seat.