I'm not sure if we're talking about the same pieces, but I think we might be. I was looking in the '69 body assembly manual the other day, and I noticed an illustration showing some kind of rubber insulator mounted to the bumper brackets. I recently blasted and painted my original bumper brackets, and whatever rubber was originally there was long gone.
It'd be nice to have a picture of an original, or some ideas of how to recreate what was there. Like you said, I really don't understand what the insulator was supposed to insulate from.