Special 1/2 inch allen hex key / spanner designed to install or remove the tails of union type radiator valves that are screwed into a radiator.
It is made from high tensile steel and is strong enough to accept sufficient torque and even mild hammering to assist removal of stubborn tails.
The end of the key is sized to go inside a standard radiator valve connector tail and it also has recesses to engage internal studs which sometimes exist in the tail. The other end of the tool has a square to remove 1/2 inch square air vents.
This IS NOT the tool you need to operate the air vent at the top of your radiator (though we do keep them too - Radiator air vent keys)