Hey J, from my understanding the original foam in '72 had some kind of spring in it, ergo the one year only foam. I actually had to redo mine because the "acquaintance" I had install mine did a God Awful job and I had to do it over. That said, I really didn't find anything too obvious as far a "spring" inside and they were Original '72 seats and foam. I do think there was 2 straight rods that ran front to back back though to keep the recessed contour aligned. I didn't have the backs apart though I did cut the hog rings off the driver back to align it better.