Need help identifying the year of this master cylinder/brake booster

Close. It's actually a '73-'75 A-body unit. The '76 boosters are different.

Make sure to get a new master cylinder from a reputable brand (Bendix or Raybestos); "remanufactured" ones are garbage. As for the booster, the only ones available through parts store are (again) "remanufactured" junk. Use Booster Dewey instead.