The major difference is......
No one makes foam for 1972 frames & up, the frames are different & the difference in foam shape is huge inside the seat back. The listing rods the cover rods are hog ringed to are also INSIDE the '72 up foam. So far, no one has taken on the reproductions.
So with 1970 to 1971 buckets, you can get new foam.
Or you can look for a set of 1972 up with great foam or at the minimum, repairable foam.