That's a good question. The answer is BOTH. There were two styles of booster. I don't know what dictated what style booster was used. Disc brakes were standard by 73 so they should have used just one style but they didn't.
The Midland Ross boosters are plated zinc dichromate. That's the yellow/gold plating. The Midland Ross boosters are the banded style. They have a metal band that holds the two halves together. The other style I believe is a Bendix. That booster has the two halves crimped together and was painted black. In 1970, the master cylinder was installed on the booster and everything was painted black as an assembly. I don't know if that is the case with the 73/74 cars or not. Someone should post a picture of a survivor car with a black booster to see if there are traces of black paint on the master cylinder.
Here are a few pics of my booster before I sent it out to Booster Dewey...
The original booster was rusty on the outside and I blasted/painted it years ago. Here's how it looked before I took the backing plate off of it for the first time.
Here is a picture of the back side of the backing plate. It was originally painted black.
Here is the area dirrectly under the backing plate. The plating still looked brand new.