Door differences.

Door handle location, window crank location, and all of the internal window crank and lock hardware is different between the 68 and '74 door. The '68 door also has a metal top, where the '74 door uses a plastic trim piece, and the interior door panels are different. And of course there's a wing vent on the '68 door.

The door will bolt in place on the car and the outer sheetmetal looks the same, but that's pretty much where the similarities end.