The 70- early 71 seatbacks don't have "arms" and have a rectangular opening for the seat release button like this.... This style requires 4 metal seatside brackets to go along with the plastic seatbacks.
The late 71-72 A-Body cars had a smaller opening for the seat release button like this. Also, the seatback has "arms", as shown in the last pic. I believe the B-Body cars stayed with the hardbacks thru 74, but the A-Bodies switched to softback in 73. I'm not positive whether the E-Body cars stayed hardback thru 74 or not ??