The rears were painted black by eaton.
Actually, the rear axle was painted at the Chevrolet-Detroit Gear & Axle plant, where it was built; Eaton only supplied the Posi unit.
My mistake on the location, was the description of masking and lack of markings correct?
I will add that the brake drums were in place when painted and had I believe a push on spring washer type of retainer, (much like the ones that hold the heater cables to their levers) the coverage on drums varied from good on some to non existent on others.
Because the drums were in place the brake side of the backing plates would have been natural steel, some restorers use high heat charcoal grey inside the backing plate etc so they look closer to correct when they pull a drum off.