You never have found one? I see an ad like yours every few months or so.
It just so happens my car is in the body shop and just had the passenger lower quarter replaced, and I am going to swap on a 72 Duster Rear Bumper on my 74 Duster, and I have all three pieces of the trim in question.
PM me and lets exchange contact info and I might trade ALL THREE PIECES for some 74 Duster Odds and Ends you may have.
I will not seperate the set though.