In the 1960‘s seatbelt laws were a relatively new thing. Until the National Traffic Safety and Motor Vehicle Safety Act was passed in 1966 there was no federal safety standards on automobiles. The law requiring seatbelts in all seats came in to effect in 1968. Cars manufactured before then generally had seatbelts as optional accessories. The front ones usually came by default and the rear belts were special order.
In the case of my ‘65 Mustang the front seats both had lap belts and the rear seats had nothing. At some point, a previous owner installed a lap belt for one of the rear seats. I’ve replaced the existing seatbelt (which was the wrong color) and installed a seatbelt for the other seat.
That almost looks like it’s supposed to be there!
Hooray for slightly more safety! This won’t, sadly, make those rear seats any more comfortable.