GttC
The French military do use a civil-style "registration" for flightplan and occasionally callsign purposes but this is not an official civil identity. These may or may not tie up with what is painted on the aircraft. Examples would be the Transalls which usually quote on flight-plans a "registration" e.g. F-RAZQ for an aircraft coded 61-ZQ, and famously their C-135s, which use "F-UK" as the first three letters. F-UKCK is an actual example, belonging to aircraft 63-12739 coded CK.