Thanks Pete! I have that same black metal drop down attachment to my bumper and attached to that is a black plastic "bar" that my plate attaches to. Very similar to yours although mine is obviously a plastic repop that is similar to your silver piece.
Anyway, it would seem that the 67 rear plates do not attach the same way as the 68's do, so my question seems to have been answered then. I see that you do not have the holes that the 68 pictures show, but I do. The previous owner of mine must have replaced that rear body piece with a repop that has the 68 mounting holes.
