I agree that the most likely answer is that it's a 50th anniversary commemorative. The dates are the obvious give-away, and the use of the "CANADA" scroll over the gun puts it before 1926.
http://regimentalrogue.tripod.com/blog/index.blog/2356317/the-royal-canadian-artillery-and-ubique/
The three sets of cypher letters represent the three sovereigns the Regiment had served under up to then.