66 seats have no release button and have a metal back and metal bottom sides.
66 seats frames were the same for Nova, Chevelle, Impala, Cutlass, GTO, LeMans.
66-67 Nova seat tracks are specific to 66-67 Novas because they are shorter in length (approx 14.25" hole to hole from front to back) and were shorter height-wise too.
So you can find a set of (just picking a car model) 66 Chevelle seats, and put the correct foam and upholstery on them, and also shorten the tracks or find 66-67 Nova tracks, and they will be fine.
On another note, you can also find any buckets from the Models listed above from 66-72. They were all the same frames, the release mechanisms were different throughout the years, the backs were different material, and the upholstery/foam were different.
So you can technically get a set of 69 Chevelle seats, get a set of 66 backs, foam, upholstery, and tracks.......and it will work perfectly.