IIRC it was much later, around 1978 it became mandatory, some states making it mandatory before 78 but it went state wide in 78 IIRC.
It wasn't 72 as the 500-4 has no provision for always on headlight, and it would have needed to have it if it was 72.
I'm in the UK and I've often wondered if bikes manufactured before then are exempt from having lights always on when on the road, or is it just something owners have to remember to do? What happens if the bulb blows, are you not allowed to ride it home, even in sunlight? I don't disagree with the principle, I'm just seeing if there are grey areas for certain situations. In the UK we are not required to run with lights on during daylight but bikes after a certain date are required to have them always on, you could however easily remove that function and it would still be legal, whether the insurance company would pay out in the event of an accident though tends to stop people doing that.