Were do you find serial numbers on a diamond arms Saint Louis co single shot 12 gauge?

Diamond Arms was a trade name used Shapleigh Hardware for guns that were actually made by Iver Johnson of Fitchburg, MA, c. 1920-1950. Prior to that, they were made by small local makers in Belgium. These were utility grade guns. Prior to 1968, serial numbers were not required on shotguns and rifles, and there are many inexpensive shotguns that have never HAD a serial number (and yes, that is legal)