The paint was applied right before the wheels were installed on the car. Some original wheels show red/orange paint drips on the back side of the wheels. My 73 drums were painted so they did it until then at least. Someone posted a factory drawing that showed the calipers were to be painted as well. However, I don't think it's ever been proven that it actually happened.

If you could see the drum through the wheel, then it was supposed to get painted. So, a Rallye wheel and a road wheel equipped car should have painted drums.