The abbreviation RCA was used in reference to those Canadian artillery regiments granted Royal status by the reigning monarch. It was not in general use. My sources do not list RCA among the abbreviations used for shoulder titles among the Canadian artillery in World War 1.
I can find no 165th regiment, battalion, or company listed among British or Canadian artillery units. I did find the 165th (French Acadian) Infantry Battalion listed among the various Canadian infantry formations formed during WW1. This battalion was absorbed by the Canadian Reserve Battalions (i.e., it didn't fight as an independent formation).
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