10.4mm converts to .4094" or about the same as the .410 designation. A deep grooved rifled bbl designed for lead bullets such as the .44WCF could very conceivably be marked with a 10.4mm bore diameter & be grooved for the ordinary .427-.429 ".44Cal" bullets.
The link on the .410 chamber was too small for me to read & when I tried to enlarge it, it bcame too fuzzy to read. Dimensions I have for minimum chamber for the 3" .410 list a dia at forward end of chamber of .463" with a body taper of .005" per inch, so minimum head dia would be .478. Thus it is easy to calculate that 12mm (.472") would fall a bit past 1" from the breech in a minimum chamber, a bit further if the chamber is above minimum. I am not certain if the dia of the shorter chambers are the same at the head or the forward end of the chamber.
