An extremely rare original British WWII No.3 Mk I pressure switch, designed for covert demolitions, booby traps, and sabotage operations. The device consists of a robust hinged metal plate with an internal spring-loaded striker mechanism. When pressure is applied to the plate (e.g. stepping on it, closing a door, or placing an object upon it), the internal striker is released, activating an attached detonator and explosive charge. This particular example retains its original protective wax wrapping around the striker assembly — a strong indicator of its untouched condition — and the integral trip wire retention system. The No.3 Mk I switch was one of the earliest and most advanced pressure switches produced by British ordnance for clandestine warfare, issued widely to Special Operations Executive (SOE), Office of Strategic Services (OSS), Commandos, and other British special units.Due to its limited wartime production and highly classified nature, surviving examples are extremely scarce.