[rgunlock]

I don't have access to my photos at the moment, but it actually does look like my seat is likely an 81-87 model. Still original GM, but not for my year. If you look at classicindustries.com, they sell seat and back foam for the 73-80 but only the seat for 81-87 which makes sense if the back foam is molded onto the frame.

Unfortunately, I already bought foam and upholstery for a 73-80 bench before I pulled off the old upholstery, both of which have different listings for 73-80 versus 81-87. I think it will be easier for me to try to find a 73-80 bench to rebuild than to try to exchange those bulky items for 81-87 equivalents.