RGA is the abbreviation for the Royal Garrison Artillery of the British Army. In WW1, this organization provided corps and army level heavy artillery (as opposed to the RFA - Royal Field Artillery - which provided mostly divisional light artillery).
To my knowledge, the letters RCA were not used in an official capacity. Canadian artillery was designated by the letters CFA (Canadian Field Artillery). I will check to verify the letters RCA were not used.
I will also look for 165th RGA. This could indicate a regiment or company. (I think the latter, but I'm not sure.)
The date, August, 1916, could place the former owner at the Battle of the Somme (July - November, 1916). Unit identification will help to establish if this is true or not.
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