It has no markings that I can find. Nothing to indicate caliber. Seems to be in good condition. I would love to know more about it, but can not seem to find anything that looks quite like this via Google searches.
You're handy to have around you know that?It appears to be a "Hero" derringer. American Standard Tool, and Manhattan Fire Arms sold these, and copies were made in Eibar. These were called "the poor man's derringer". They were ludicrously cheap .32 to .36 caliber pistols.
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Brass ones in this condition go for between $50 and $70, and steel/iron frame ones go for $70 to $125.
Unfortunately, yours appears to be a Spanish clone.