I have DA170, and DA386.
That is a mark applied by the Chilean navy.
Prior to WWI, the Chileans were having a battleship built in Britian. The war started, and the Brits took over the vessel, finished it, commissioned it as HMS Canada. HMS Canada was at the Battle of Jutland in 1916. IIRC, fired 45 rounds from its main guns. HMS Canada was RN, not RCN.
Anyway, your Mk. III probably went to Flanders with the CEF. After the Rosses were withdrawn from front line issue, they were relegated to other service. Many went to the RN, including yours. After the War, HMS Canada went back to Chile as the Almirante LaTorre. The Rosses went along as ship's stores. The Chileans scrapped the ship in 1958, the rifles were eventually imported by Century Arms in Montreal. They offered the rifles with matching bayonets, rifles and bayonets separately, and sporterized rifles. So that is how your rifle got back to Canada. If you have the bayonet, it will have the matching Chilean serial number, RN pattern frog, and RN pattern blade regrind, which is different than the Cdn. regrind. Scabbard and frog will be black, not brown.
Look at the marks on the right of the butt. The CEF marks should be there. If the serial is before 1916, the rifle may well have been involved in the bitter fighting in the spring of 1915, 2nd Battle of Ypres toward the end of April, and the following battles.