Brake master possible leak

The photo helps a lot. Shouldn't be a seal. I have a similar Bendix booster on my 65 Newport. I have also had Midland-Ross boosters (V-band clamp). All have a plastic piece with a "drain slot?" at the bottom. You can unbolt the MC and pull away slightly to see, won't hurt anything.

You are correct that some cars do have a vacuum seal between MC and booster. My M-B are like that w/ a thin Girling booster. The boosters on my minivans are similar thin, pancake type. The boosters on our cars have an internal bellows around the output rod that seals vacuum. I suspect all thin ones don't and thus must use the MC to seal vacuum at the output hole.