The only real difference is the cut out for the bumper. On the 70-73 cars there’s a flange where the quarter is joined the thr tail panel, the quarter has the bumper relief stamped in and then the flange sticks out and covers the gap between the bumper and the tail panel. On the 74-76 cars the quarter just ends there, it has a 90* corner that just overlays thr tail panel.
You can tuck the early bumper in on the 74-76 cars just fine, but you can see around the end of the bumper because there’s no flange. The 74-76 cars just had that plastic filler piece because the bumper stuck out farther to account for the shock mount bumper brackets.
I’ll see if I can post better pictures of the difference in the quarter.
