Switch Number 3 Mark 1

No 3 and No 3 Mk1 MD1 Release Switches (from John K. Neuenburg)
No 3 Mk II and No 3 Mk1 MD1 Release Switches (from John K. Neuenburg)
MD1 No 3 Release Switch (from John K. Neuenburg)
MD1 No 3 Release Switch (from John K. Neuenburg)

Switch Number 3 Mark 1 – Release Switch. 

Placed inside or under something, this switch operated in reverse to the pressure switch. When a weight was removed from it, or the closed part of the device released, for example the opening of a tin, this triggered the explosives connected to it. Often this was used by the army for booby traps. In the images the one with the added coil spring is Mark II.

References

John K. Neuenburg