Odds are it's 9mm, though 7.65 mm/.30 Luger is a possibility. Pencil test! A standard pencil will plunk to the bottom of a 9mm bore with no resistance. A .30 Luger barrel will be a snug fit. (Don't use the end with the metal collar.)
9mm--Military, almost all Police, some commercial.
.30 Luger--mostly commercial and contract guns. The Swiss used this caliber from 1900 until the Parabellum was replaced as their standard sidearm.
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"... Liberty is the seed and soil, the air and light, the dew and rain of progress, love and joy."-- Robert Greene Ingersoll 1894
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