It is a Kerner-Anson platform that was probably a 16 over something that was bored out to 0.410" The stamp of >>16X410<< is a odd stamp more than likely applied in the U.S. of A. There should be additional marks to ID the original kalibre.

Yes, March of 1926 could be the date of proof.

Serbus,

Raimey
rse