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.

Jailers’ Key Guns (Oobject)

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