Source: Per post at 21st Battalion Facebook Group.
Find-A-Grave
This soldier originally attest to the 21st Battalion but was transferred to the 18th Battalion on August 1, 1915. He served with the battalion until September 24, 1918, when he was TOS to attend an officers course.
More information at the 21st Battalion web site.
War Diary of the 18th Battalion CEF 