I have a hammer Peiper "Cape Gun" with one barrel marked ".44 Largo" (Spanish designation for .44 WCF) and the other (smoothbore) "12mm." The "12mm" is chambered for the 2" .410 shotshell. Don't see "10.4" marked on it anywhere, but 10.4 mm is the European designation for barrels of .427-.429 size elsewhere, for example in Italy.