I researched this subject for my V code ‘71 Challenger, and what I have found is that there could either be a black painted Bendix booster, same part as used in ‘70, or a cad plated style made by Midland Ross. I’m not sure if both were used all year, or if the Bendix was used earlier in the production run and then was transitioned to the Midland Ross later in the cycle.
The master cylinders used starting in 71 were finished in a gray parkerized coating that resembled bare steel, though that’s based on NOS ones I’ve seen, I don’t know if 1971 production era master cylinders were still painted black like disc brake Mopar MCs were in ‘67 through ‘70.