I'd say it was black, then sprayed over afterward with OD for the military.

At least with most vehicles I have looked at, anything exposed seems to be painted OD. Maybe a military requirement that any visible surface had to be OD??

My frame was black and then oversprayed with OD, and it appears that it is original that way. There are spots under the truck that look as if they couldn't get to them easily, so they left them black.