The only long service award that could be earned by a member of the RCN before the institution of the Canadian Forces decoration was the Royal Canadian Navy Long Service and Good Conduct Award (RCNLSGC).* Only sailors and petty officers were eligible. There was no good conduct and long service award for officers because "they were expected to act with good conduct at all times." Eligibility for the RCNLSGC award was instituted in 1925 and it required 15 years service (in context it was identical to the RN LSGC Medal).
Before the adoption of the RCMLSGC Medal, sailors and petty officers were technically eligible to receive the Permanent Forces of the Empire Beyond the Seas Long Service Medal. This medal was awarded to non-commissioned members and there is no record of it being awarded to any member of RCN.
Based on the above, I suspect the following may be the case:
Since officers of the RCN did not qualify for any Long Service medal before the institution of the Canadian Forces Decoration, it is possible that prior service could not be carried forward to the CD. The terms of reference for the CD state that eligibility starts with service on or after 1 Oct 1946. With a 1 Oct 1946 start date for eligible service, for those recipients whose earlier service did not qualify towards a long service award, the earliest award of the CD would be 1 Oct 1958, and the earliest possible bar earned on 1 Oct 1970.
This last part is not correct. Qualifying service for the CD did not start on 1 Oct 1946. The member simply had to be serving on 1 Oct 1946 (i.e. a guy with at least 12 years of service who retired on 30 Sep 46 was not eligible, but if he stuck around until the next day, he would be eligible). The service of regular force officers prior to 1 Oct 1946 was not excluded from the CD, even though it had not been eligible for any other LS&GC. Case in point: VAdm Rollo Mainguy, OBE,
CD retired in 1956, only ten years after the institution of the CD. If his prior service had not counted, then he would not have been eligible. In fact, if you go to the main lobby of the HQ building in Halifax, (D201 - the Mainguy Building) his medals and decorations are on display and his CD is the George VI version, meaning it had to have been awarded prior to 1952 (or thereabouts - the change to the Elizabeth II version was not instantaneous).
Article 6 of the CD regulations (1949 version):
6. Service Required – Royal Canadian Navy
– Canadian Army (Active Forces)
– Royal Canadian Air Force (Regular)
(a) Twelve years full time paid service in the Naval, Military or Air Forces of the
British Empire, provided the individual was serving on or after 1st October, 1946 in the Royal Canadian Navy, Canadian Army (Active Force) or Royal Canadian Air Force (Regular).
(b) Service in the Reserve or Auxiliary of the Armed Forces of the British Empire, other than full time Active Service will not count as qualifying time.
(c) Effective – (date of promulgation) – personnel will cease to qualify for The Naval Long Service and Good Conduct Medal, The Canadian Medal for Long Service and Good Conduct (Military) and The Royal Canadian Air Force Long Service and Good Conduct Medal.Although service as an officer did not count toward the LS&GC, officers who had earned it as non-commissioned members could keep it and continue to wear it.
Since the OP stated that his ancestor had started in the ranks, the award of an LS&GC could have been possible.