It may have belonged to a Mountie, under circumstances we can’t reconstruct, but the issued service gun for the RCMP from 1904 to 1919 was the Colt New Service in .455, with the 5.5” barrel. Starting in 1919, the same gun in .45 Colt was issued. The New Service remained on duty until replaced in the mid-1950s.

The crossed-pennant proofs indicate that your gun, no matter where it entered Allied service during WW I, started its career as a military, not a police gun.