Maybe the rubber "bumpers" for lack of a better term. They mount to the body ( underside of rear body panel) with 2 screws to each pad. They're at the very end ,left and right. Just above the bumper.
I want to replace the rear bumper on my 74 with the same bumper as my 70 2nd photo. 74 has the small extension shown on my 70 cut off for the grey rubber filler panel.
I thinking if I have a good condition 70-72 extension I can either weld it in, or use the one you have to make a template, and have someone make one on a 3d printer.