Duster Seat belt question

I'm trying to find some factory belts for my Duster ("IT") and have seen some from a 73. But it looks like the ones from a 73 are made with a shoulder belt that has it's own retractor and it permanently attached to the lap belt.

Were these shoulder belts mandated for 73? How does this setup fit in the car? Can they be retroed in to a 72 without trouble if a 72 didn't have them?

Need answers fast please.

Thanks