The fellas over at the Great War Forum have supplied me with this iformation:
That is the style of Service Dress (SD) collar worn by both, commissioned officers and warrant officers, from the introduction of SD in 1902, until modification to the collar was made to facilitate the wearing beneath of a soft collared shirt and tie, in 1908. After 1908 it continued in use by the warrant officers of infantry and cavalry only until around 1922 (+/-). The fifth (uppermost) button was removed as no longer necessary after the rever collar was introduced in 1908.
The open neck collar was promulgated by AO279 of 1913.
As the shoulder straps are not detachable (1902-1904) or twisted cord (1904-1913), I'd say it's just around 1913 when the switch to sewn down shoulder straps took place (and before the collar change or the Canadians caught up with the regs!!).
Shouldn't have had shoulder titles fitted, strictly speaking, but maybe that's a Canadian thing.
Not unknown to see these style tunics in France in 1914, as the scramble to add manpower fast got underway.
And 1921 or 22 was when, if I recall correctly (can't lay hands on the order), was when cuff rank itself was formally abolished.