On 1966 Charger and Coronet, optional bumper guards lined up with the hood and trunk opening seams. Since there was no convenient bumper attachment hole here, it meant providing 2 extra holes in each bumper face for the guards. I imagine Chrysler would have to stock separate bumpers for each application. Merely drilling a hole in a solid bumper would lead to eventual failure in the chrome plating around the drilled hole. In 1967, Dodge decided to mount the bumper guards on existing bracket attachment holes. This required new/different bases. Looking at original, period pictures, the rear bumper guards were mounted on the outer bracket holes to provide maximum protection from parking mishaps. My 1967 Coronet R/T had its original bumper guards when I bought it, and the rear guards were attached this way. I thought they looked dorky this way, and I moved them in-board on the other bracket hole. This more or less balanced the look of the front bumper guards placement. Looking at pictures of restored '67 cars, just about everyone has done the same thing for better looks.
I have part #'s from 1967 Mitchell's collision repair manual:
Front bumper guard 2768732-3 (note R. & L.)
Front cushion 2768636
Rear bumper guard 2768751
Rear cushion 2533494
Judging from much lower part # , I am guessing that rear cushion is a carry over from 1966. Saying that, the only pieces that would interchange from 1966 to 1967 would be the rear cushions. However, if total originality is not a factor, the 1967 pieces will bolt to a 1966 bumper, using the bumper bracket holes, since the general shape of the bumpers are the same. The bumpers are different in terms of signal and back up light placements. Note that there is a definite right and left 1967 front guard because of bumper contour. If you get them on backwards, you will notice it. You can possibly access 1966 Mitchell's manual online to get 1966 part numbers. Hope this helps.