Remember that offering is not quite the same as available. From your link, it seems that Ford offered this as a package starting in 1956 for some models and it did not sell well [1]; some accuse it of making a half-hearted effort. However, I have to say that kudos to Ford and Robert McNamara for trying to sell safety and offering safer options well before others.
1968 was apparently the first year that seat belts were mandated in cars.
It had 2 seatbelts in the front, and none in the back. It was a bench seat in the front though, so we had 3 in the front and 4 in the back.