I was kind of wondering the same thing - thinking along the lines of the bottle caps (chocolate milk, IIRC) used to fill the holes left when people have gone bumperless. But then it hit me - the caps on the end of the bumper rails. On the '76-'78 US models, they're the big rubber things that attach to the end of the rails and wrap part way (not very far!) around the corners of the car. On the '75 it was a little smaller and the '74, well, yeah. Unless my thinking is WAAAAYY off again...