If you hold a second mates license and sail as a third, doesn’t 6 months of that count toward your CM upgrade?
360 days of service as Officer In Charge of a Navigation Watch (OICNW) on Ocean/Near Coastal steam or motor vessels while holding an endorsement as 2nd mate.
You can find this information on the USCG National Maritime Center website www.uscg.mil/nmc Go to the checklists tab which is halfway down the main page underneath the photo. C/M checklist is there.
All your time can be as 3m while holding a 2m license.