Most of the center springs are the same I believe but the side springs differ. I had to replace a couple as some of mine were completely gone. I ended up finding a std back seat and raping the springs out of it. I had to reconfigure some of them(cut turn upside down) and fit them into the seat but it worked. I'm not sure you can buy rear seat springs.
Mine had burlap over the springs and then a layer of cotton batting(3/4-1"thick) and the the cover over that. Pretty sure that's the way they were made.