Hi Uboatfan,
I suppose these are markings to recognize the single boats from an airplane, maybe for training tactics to attack a ship convoy (?). So the letters are the name of the boats for guiding them by radio, I guess. The Baltic Sea (Polish coast) was for a long time out of range for allied airplanes, so it was a safe training area. Maybe the "Rudel-Tactic" was first trained on the water by conducting the letter named boats from above – just an assumption.
Regards
Falo