Red, looking inside for impressions and other clues is always one of the first things I do with mystery holsters like this and usually I'm able to figure out the gun this way but this one shows only long rectangular witness wear from an unknown semi-auto slide. Futhermore, the holster body itself is very flat and square (rectangular in shape looking down into the mouth whereas holsters for revolvers are typically oval in shape from this same angle) with no boning for a revolver cylinder. I could probably come close to figuring out what gun it was actually made for by researching which older pistols were roughly the same size/shape as a modern Glock 43. It may be for an Officers model 1911 or something similarly smaller.

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SWHF #448