baltic sea year-excursion 1931, return flight, stage Luebeck - Friedrichshafen, letter from Luxembourg 12.5. with colourful airmail mixed franking to Luebeck to the return flight to Friedrichshafen, from Luebeck 14.5. in two countries mixed franking with 2 Mk in colourful airmail mixed franking, flight stamp on face, arrival Friedrichshafen 15.5, in this variety as incoming line not listed