Three layers, a bump in the middle layer for leverage. Simple, effective design.
You know – for keys.
These vintage devices are sometimes confused with early spy gadgetry but the truth is much stranger. Sez David Galbraith of Oobject:
Jailers’ keys were apparently filled with gun powder to create a primitive gun that could be detonated if there was any trouble when opening a cell door. We found several original versions that back up this claim, dating from the 17th century and of various complexity.