Seat belt laws?

I have a 67 and think that was the last year you only needed "Lap" belts - that or 68.
If you have a 72 I would think you need the "Shoulder" belts also. A web site that sells
seat belts should have that info. Think it came with the Lap belts I ordered from some
site on the web. Thought seat belts was a Federal Law.