I believe at some point the retractable mechanism was recessed into the rear panel, not on exposed on the floor. Perhaps this was in 73 when they changed the interior panel design to plastic??
It was '74 when they changed it. Still had a retractor on the floor for the lap belt segment, but also a retractor recessed into the body next to the rear seat bottom for the shoulder belt section. So 2 retractors.
I thought about trying to add the structure for the recessed retractor, but not sure how difficult that might be. Doesn't seem like it would make for a difficult retrofit since there is just a big hole at that location in the '73.