If it's really a Model 12, there ought to be a serial number on the bottom front of the receiver, with a matching number on the barrel-mount. They only made several hundred in 1912; I have a 20-gauge, 14xxx made in 1913.
A plain vanilla Model 12 with poor finish but not beat up and banged around will bring $300 and up. Better, quality, better grades are worth more, of course.
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