I would be more inclined towards a British regimental marking, and might well stand for the Cheshire Regiment ( martinihenry.com / metal markings/ regiments )
CH. The Cheshire Regiment 22nd Foot
The UK Camerons were apparently identified as:
CAM. The Queen's Own Cameron Highlanders 79th Foot
The only Canadian regiments sharing the titular honour, and allied with the original Camerons were the Cameron Highlanders of Ottawa and the Queens Own Cameron Highlanders of Canada, from Winnipeg.
The CHofO, at the time of your rifle would have been in service, would have been a rifle regiment ... either the 43rd Ottawa and Carleton Rifles or later, as the 43rd Duke of Cornwalls Own Rifles. The nomenclature CH on the buttplate could not indicate the Ottawa regiment, as it only became tasked Highland Infantry in 1920 or so, and thence the Camerons name.
The possibility of service with the Cameron Highlanders of Canada, though, is possible... although at that time a fairly new regiment, the regiment's abbreviations could have been CH for Cameron Highlanders, but if memory serves, the short form for this regiment was CAMERONS or CAMERONS of C ... and thus the letters CC would probably have been used on the buttplate stamping.
If a Canadian issued rifle, there should be other markings indicitive of this. During this period, the C-Broad was well in use and would almost certainly appear somwhere on the rifle. We await further pictures with bated breath!
All in all, an interesting rifle that certainly warrants a lot more reserarch...