I'm definitely a Martini lover!
I've got three Martini-Henry rifles in .577/.450 (a Mark I upgraded to Mark II, a Canadian-marked Mark III like yours, and a Mark IV) as well as a .303 Martini-Enfield carbine and a .22 BSA Canadian-marked cadet rifle.
The "DC in diamond" stamp was the Canadian government acceptance and ownership mark adopted after Confederation - denoting "Dominion of Canada", of course. However, as has already been alluded to, unlike the rest of the British Empire Canada never did adopt the Martini-Henry as a primary-issue military firearm. Because of the Fenian emergency, Canada got preferential treatment in being provided with the Snider-Enfield breechloading conversion adopted in 1866, with the first several thousand rifles being received in 1867, and about 60,000 in total by 1875. Snider-Enfield rifles and carbines remained the primary-issue longarms of our military forces until adoption of the Magazine Lee-Enfield rifle ("Long Lee-Enfield") in 1896.
Canada did acquire 2,100 Mark I Martini-Enfield rifles in 1871, but other than a hundred or so issued to the Royal Military College, and some others to Militia Rifle Associations (so they could compete with other Empire shooters with the current British military rifle) the arms all remained in Stores. When the 1885 North-West Rebellion broke out, Canada hurriedly ordered 10,000 of the latest pattern Martini-Henry, the Mark III. Our troops went into the field armed with the Snider-Enfield, however, and the order was later reduced to 5,000 M-H rifles, the majority of which were also never issued. Your rifle is one of that batch, as is my Mark III which is shown here (with an 1889 photo of members of the Queen's Own Rifles of Canada rifle team as a background) -
I shoot my Martini's (primarily my MkIII) quite a bit. At the last two Annual Musters of the Grand Army of the Frontier (an organization which melds cowboy action shooting with late-19th century military history and re-enacting) I have competed my MkIII M-H in the Single Shot Battle Rifle category (along with my 1890's Webley 'WG Target' revolver) and have placed first both years! The most recent occasion was just over a month ago in Nebraska - and FWIW here are a few photos of me in competition (the uniform is circa-1885 Queen's Own Rifles) -
Since you are located in Alberta, you'll want to consider joining us at the annual "Alberta Old Gun Shoot", an informal shooting gathering and "playday" which is held every summer west if Inisfail - it is an outgrowth of the British Militaria Forums someone has already posted a link for - here is a direct link to the discussion about the 2008 shoot (with losts of pictures starting about page 7) -
http://britishmilitariaforums.yuku.com/topic/2331
And here's a link to the discussion already happening on the 2009 gathering -
http://britishmilitariaforums.yuku.com/topic/7535