A tough call. Usually the cadet formations did not use the letters RCN (Or RCA in the case of army or RCAF for the air) but rather finished their titles with the C, as in RCAC (air cadets), RCACC (canadian army cadet corps) or in the case of the sea cadet formations, Royal Canadianicon Sea Cadets, or RCSC.

According to Wikopedia, the HMCS Ontario was commisioned too late to see any action in Europe, or the Pacific, but sailed on until 1958. Very possible your rifle could have been LongBranch FTR'd and then issued over to the RCN and attached to this cruiser.

HMCS Ontario (C53 - Wikipedia, the free encyclopedia)
She was used for training duties postwar until paid off on 15 October 1958. She arrived at Osaka for breaking up on 19 November 1960.
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