Rear end housing color....

Hey everyone!. I am restoring a 1971 Plymouth duster 340. My uncle has been the 2nd owner since 1973. I was documenting a few things and as I was underneath I noticed the whole rear end housing is covered lightly with black undercoating. But underneath the black undercoating is the body color gy3 curious yellow. I asked my uncle if he ever painted the rear end housing and he has not done so. Checked the broadcast sheet and it has undercoating from the factory. So I went back under, pulled off a piece of the paint flaking off and underneath the yellow is bare metal. Did the rear end housing ever come painted body color? Car has been parked since 94 and hasn't moved