In it's OEM configuration (as produced from the factory), any MC in an early Mustang with drum brakes would have had a residual pressure valve (RPV) under the seat, in the MC port(s) supplying fluid to the drums.
If you go to a parts supply place to buy a replacement MC, chances are great that the replacement MC isn't going to have any RPV(s) installed in it, even if the new MC is a direct replacement for the OEM application.
You can insert the shank of a small diameter drill bit into the hole in the MC port to see if a RPV is present. It's just a small duck-billed device. If it's present, the drill bit won't go very far into the port. If it's not present, the bit will go a pretty good ways into the MC port.