There were a few different ways of attaching these things. Ford, GM and Chrysler each had their own method of connecting the lap buckle to the shoulder belt.
My father had them in his '73 Plymouth Fury III. I think that this was the first new car that he bought that he was really proud of. It was a big, impressive car that he bought moments before the first gas crisis.
The funny thing is, he always made us wear seatbelts, but we only used the shoulder belts when we were going on a long highway trip, like Connecticut to New Hampshire. Connecticut to Bergen County, New Jersey usually just merited a lap belt, and maybe a shoulder belt over the Tappan Zee Bridge.