My great grandma's cousin, Frederick William Stocks, reg. no. 18570, (born 1896/99- in Thorpe Hesley, South Yorkshire, England, son of Robert Stocks and Helen/Eleanor Matthews) fought in World War 1 as a private, but was killed in action on 22/08/1915 at Dardanelles (a narrow strait in northwestern Turkey connecting the Aegean Sea to the Sea of Marmara). On his service records (under the heading Promotions, Reductions, Casualties), it says 'Killed in action. Dardanelles.' and then it gives this; 'M.5609'... what does 'M.5609' mean?

I don't know much about the military so I wondered whether this was a code for something? I tried a simple Google search but came up with nothing. Could anyone shed any light on this?

Thank you for any help. I've attached a picture of the timeline featuring this number to this post.