Your picture shows a Bendix(crimped together) booster. Black is correct for that booster. The Midland Ross booster is by far the norm for 73. I've seen a few 73's with the Bendix. My guess is that there was some sort of supply problem with the Midland Ross boosters and the Bendix needed to be used in its place. What is your car's SPD?
As far as what black is correct. As far as I know, Chrysler NEVER specified that a part should be semi gloss black. All black parts were GLOSS. However, the paint they used was very cheap gloss black. The color was never as glossy as what we have today. That's where all the confusion started. Some parts were "dipped" in cheap gloss...others were "sprayed". It's possible that the "dipped" parts had a different gloss level then the "sprayed" parts, but Chrysler never intended them to be that way. I had my air cleaner sprayed gloss black. The pulleys and K-member were "dipped" from the factory I believe. On those parts I used a semi gloss. Eastwood's Extreme Chassis Black(satin) is a great color and has the correct look IMO.