There's a couple differences between the 71 and 72-on at least. Foam shape is pretty identical between 71 and 72, but I can't speak for what they look like compared to the year you have. The overall shape of the pan is the same. The difference is how the cover attaches. 71 and earlier have metal "teeth" that grip the cover, and the chrome trim is a separate piece. 72 and later, the factory cover had chrome trim attached to the vinyl, and this was held on place by larger blunt tabs.
The issue arises because this is really hard to replicate on new covers, so most reproductions call for you to somehow attach the cover to the pan in another way (glue, etc.), then cover the edge with decorative chrome edge trim. It looks okay, but is a bit of a pain compared to pans with the teeth.
Another option is to use an early pan, but the seat latch hardware is different (and not easily swapped over to the later catches), and all early components are quite a bit more expensive than the post-1972 stuff.
