WOW, I was off by a many years:
Seat belts have been mandatory equipment since the 1968 model year per Federal Motor Vehicle Safety Standard 208.
As far as my parents were concerned, they were mandatory since the early 1960s. I remember my father insisting about 1961 that the new car he was buying had to have seat belts. The dealer didn’t really want to install them, but Dad declared he would not buy the car unless ALL the seats had seat belts.
older cars were grandfathered in.
a lot of states never mandated the use of rear belts for a long time.
It was up to parents\driver to see to the use of...what a novel concept.