The air recirculate valve on the early discovery on the heater used vacuum source to operate it,
This came from the T piece you showed, it probably had a blanking plug over the end from the factory that has been lost so substituted with a screw, ( probably Land Rover being cheap and using parts for two vehicle types)
It won’t be for a vacuum gauge as it won’t tell you anything about the engine only the amount of vacuum for the brakes, ( useless info)