Beginning in 73 the fold down rear seat was an option for the Duster and Dart Sport, the bars you have saw are exactally what you thought, braces for the non fold down seat, the fold down seat is made stiffer because of the access to the trunk, lots of cars came with it, I have had about 6 or 7 Dusters and Dart Sports and it's been about 50/50 for me, my 74 Sport has fold down