British made and Birmingham proofed percussion key pistol. Approx. .45 cal. Made 1850 – 1860.

Some of the key guns were very basic, requiring the jailer to use a a cigar, cigarette or match to ignite a touchhole, while others used sophisticated percussion cap and flintlock mechanisms.

Ian’s video discussing a similar design: [3:49] https://youtu.be/kTSaZAOCla0?si=

For some other ones: https://www.thefirearmblog.com/blog/2011/07/05/key-guns/