Very unusual construction (the belt loop is folded inside against the flap; rather than attached to the backside separately). The very fine stitching suggests perhaps 1900. Sewing machines were most uncommon even then.

For sizing I expect it's for an SA of some sort, which could be a broad spectrum of things in that era. Take thee to a gunshop that has antique-era pistols.

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Red Nichols The Holstorian