Thanks Alan and Smallonion. According to one of my contacts it should be read as FD6 which is the map quadrant of a German Gradnetz-map system. I have a number of aerial photos of Luftwaffe airfields and this is the only one which has the Gradnetz-map system quadrants indicated. I am guessing that they had them early on when things were going well and then with the Allied strategic bombing successes it was probably not considered good form.
All the best
Scott