I think that the "Proof Markings" were usually applied when these guns left service and were sold into commercial channels.

.455 Commercials were not always sold into the Commonwealth countries... this .455 HE1st New Century (AKA: "Triple Lock") was shipped to Shapliegh Hardware just after Christmas, 1917.









As Mr. Jinks indicates, it was a contract over run that had not shipped prior to the British modification of their contract that resulted in the .455 HE2nd...

Drew