There is a factory drawing from the 69 model year I think that shows what part of the drum got painted and on what cars got the paint.
ONLY cars with "styled" wheels got the orange paint. That means on E-bodies, ONLY cars with Rallye wheels and Road Wheels(magnum) got the drums painted. With other wheel options, the drums weren't visable, so there was no reason to paint them.
The drawing also shows that NO paint was allowed on the face of the drum where the wheel would come in contact with it. That means no paint around the wheel studs. The drums were only painted in a 2" band between the mating surface and the edge of the drum. Obviously, the line workers did this very sloppily. Some cars got more paint then others.
By the looks of your drum, I'd say that isn't factory orange paint since the entire face of the drum is painted. I suppose it's possible though.