It's earlier than 1923 mfg. The shotgun was probably serialized in 1920 because I have one slightly higher in number that was serialized that year.
But what bothers me most about the gun is that no WWI Military M1897 riot gun (at least to my knowledge) has ever been authenticated with hand stamped "US" and Ordnance flaming bomb markings. Only some of the WWI trench guns were so marked. The WWI riot guns are usually found unmarked or with a factory applied small "U.S." mark on the left rear side of the receiver above the trigger guard. And, the "U.S." marking on the subject shotgun looks a bit suspect to me. I would have to examine the gun in question to be sure. Also, the fact that it was made post WWI is troubling too.