Very nice! That is a .38 Double Action Perfected. The thumb release on the side of the gun is said to be policeman insurance for lawmen holding a suspect at gunpoint. A fast thug could reach over the top of the gun, lift the latch and push down, rendering the gun both unfireable and unloaded at the same time. The extra latch, which anticipates the cylinder release of the later hand ejector models, prevented that.

Yours was made about 1912-13. Collectors like these.

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David Wilson