Apparently there are two different bumpers in 76. Looking for the one with 12 holes not the 10 hole. Two extra inboard holes from the factory on the bottom closest to the license plate.
I have my original back one for sale, but its a driver at best... [FOR SALE] - '75 Duster back bumper
You do need the extra 2 holes for correct mounting of the guards in 75 and 76 although they will work in either position on the rear. I got a nice 74 that did not have them, and just drilled them out.