This won't be a lot of help to you in your search, but .....
On page 126 of Marcot's book, he reports that 300 56-50 Spencer rifles and 30,000 rds of ammunition were purchased in 1866 for issue during the Fenian invasion.
"Shortly thereafter, the Canadian government purchased an additional 1,000 Spence M1865 Army rifles, and by the spring of 1867, over 4,000 Spencer repeaters were on hand. The number included about 2,000 M1865 Spencer carbines, which were soon issued to twenty-two troops of Canadian volunteer cavalrymen (77 officers and 1,052 men) and to artillerymen."
"Officially, Canadian firearms were not to be marked or stamped in any manner. However, the presence of British W->D markings has been noted on the right buttstock of several Spencer rifles and carbines."
These were almost certainly Burnside produced models.