Where is the serial number on a Mossberg 12 gauge shotgun?

Typically they are stamped on the receiver of the auto and pump action shotguns either on the bottom or the side. Low and moderate priced bolt action shotguns produced in the 1940's and 50's may not have had serial numbers. Later bolt action guns had the serial number stamped on the receiver, usually the side.

Serial numbers were not mandated until 1968. You may not have one.