That would be "Flight Mechanic" (normally engaged in working on aircraft, or rather aero engines, but could also be posted to marine craft, to mind the engines installed in them, some of which were modified aircraft engines (such as the Napier Lion). MBC was the official abbreviation for "Motor Boat Crew", the trade name for most personnel manning marine craft; other special posts would include Coxswain (Master), and wireless operator, and perhaps a few others, including a sick bay attendant. MBC were generally lowly Aircraftmen holding no rank, and employed on normal crew duties, as on any small craft or ship, but could also be used to man any guns should they be installed.
David D