Question about seat bums

The original seat buns have an inner square section, and the outer curved part, bonded with cast in cords. The seat covers attach via rods that are hog ringed to base support spings. The rods go inbetween the square section as outer curved foam. I also seem to remember muslin fabric cast into curved foam edge parts, they serves to tie down and restrain. I wonder if aftermarket buns have these features?