It was the changing federal laws. First the law required side marker lights, then the law was changds so it had to be a light and reflector.
1967 and earlier have none.
1968 has the small round lights (no reflector).
1969 and later have the square lights (with reflector).
As IC noted, lots may have happened since the cars were new.
NY state inspection requires that the sidemarkers be present, but the light doesn't have to work. The laws were changed at some point that they could be a reflector instead of actually having to light up.
I used to wire the front ones to blink alternately with the turn signals like GM cars. It was an added attention-getter and that is what you want from the side of the car at intersections.