No1Mk3 Cap positions

In this position the cap can be removed for insertion of the detonator

When the detonator has been inserted the cap is replaced and turned to this position

When ready to throw the cap is turned to the fire position and the safety pin is removed. It is now armed

On impact with the ground (or the side of the trench!) the cap is pushed down and the internal needle is driven into the detonator exploding the grenade

Here you can see the small leather tab that passes through a slot on the end of the safety pin helping to keep it in position. The string loop can be looped over the cap as an extra safety feature.