so I have looked at this alot, we know it is clear they had mk11 in Englandicon prior to Franceicon,
England 1914-15 - 15th Batt Cdn Expeditionary Force
England 1914-15 - 15th Batt Cdn Expeditionary Force

Now in the french trenches. Look at the ross on soldiers lap, no mag housing.
Western Front 1915 - 15th Batt Cdn Expeditionary Force

Another french trench picture, two ross rilfes the MkIII is there as well as a ross without the mag housing which would have to be a MKII !

Western Front 1915 - 15th Batt Cdn Expeditionary Force


Western Front 1915 - 15th Batt Cdn Expeditionary Force

The last is role call after the gassing at Ypres, the lone man facing the ranks, you can see the distinctive glint of the MKII magazine spring bar above his forearm. There are M10 ross in the ranks as well as mKII.
I feel this shows the mkII remained in service with the mkIII until replacement by SMLE later in 1915