EDITED: On my tiny phone screen I had confused your gun with RS2000CUSTOM's gun shown before it, see changes in my response below.

Wow, that’s a beauty and in great condition. It doesn’t appear to have any military, proofing or import/export stampings, The two stamps it does have indicate it was sold on the Canadian market the sold out of inventory. It’s a .455 Hand Ejector 2nd Model intended for use with the.455 MKII cartridge but with chambers reamed at the factory for the longer MKI cartridge as requested by the British contract. Of course you knew that since that’s what you load and shoot in it. The serial number would indicate it shipped around the end of 1915. The British referred to this 2nd model as the MKII and stamped that on the frame of those issued to the military.

__________________
Jim
S&WCA #819