To me the single code letter suggest the photograph was taken whilst the aircraft was attached to a communications unit.
Unlike Fighter and Bomber Commands, Coastal did not always display the two letter Squadron code. It was common to display only the letter identifying which of the Squadron's aircraft it was. Where there were two or more Squadrons based on the same airfield, a number would indicate which Squadron on the airfield it belonged to, i.e. 2 F.
I never understood why Fighter and Bomber Commands gave the enemy an indication of which Squadron the aircraft belonged to. They must have known all the codes, which were rarely changed, and it gave them an insight into the Order of Battle.