Here for your perusal is Canadian WWI contract .455 (still in orginal chambering) S&W H-E pistol serial number 17464. Any help in deciphering the unit designation on backstrap appreciated.
I would agree that the markings at the top of the backstrap are army unit designations. Canadian forces in WWI initially consisted of the 1st and 2nd Divisions, formed in 1914, with regiments in those divisions made up of provisional units named for their province of origin. So a good guess for "1. H.R. 2." might be 1st Batallion, Halifax (or possibly Highland) Regiment, 2nd Division. At least that would be a good starting point for more in-depth research. Great historical piece, good luck with it.
Bill