Brake booster question

You have the Midland-Ross booster (V-clamp). I took one apart from a 74 Dart slant. It had the bellows seal around the output shaft, so shouldn't need a vacuum seal between booster and MC. However, that missing gasket looks like it is intended to seal vacuum. The similar part in my booster was hard nylon, with a lower drain slot. Thus, yours appears to be a different design. Perhaps it changed mid-year in 1974. Anyway, when you remove the MC, you should be able to tell if there is an internal bellows between the output hole and the vacuum hose port. If open, then you certainly need a vacuum seal between booster and MC.