515938 is the serial number. 70765 is a work/assembly number, which is of no relevance after the gun left the factory. The B stamp stands for "Blue" to indicate the gun is to have a blue finish. Guns with close serial numbers shipped in the mid 1920s. The grips will have the serial number stamped on the back side of the right panel, if they are original to the gun, which appears so from the photo. Ed.
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