The first 30,000 Canadians to go overseas were only issued 22,128 MkIII's which had to be augmented by MkII**, MkII 3* and even some MkII 5* so yes, your rifle could be legitimately stamped with CEF. All you need to do is find out which regiments were in the first group to ship overseas. I was thinking S.R.R. could be Soo Rifle Regiment but their battle honours didn't begin until 1917 with Arras and Ypres