An original British WWII No.4 Mk I pull switch, designed for use as a booby-trap trigger or sabotage initiator by Special Operations Executive (SOE), Office of Strategic Services (OSS), Commandos, and sabotage units. The device consists of a small cylindrical brass body with a central striker mechanism, retaining its original trip wire wrapped around the device.When tension is applied to the cord, the striker is released, setting off an explosive charge or detonator cap. These pull switches were highly effective for delaying or remotely activating demolitions, ambushes, or traps, and were a standard issue item in sabotage kits. This example is clearly marked No.4 MK I F87 AA, identifying its pattern and lot.. Due to their covert use, relatively few of these devices survive in this condition.