Austrian Flight 1931, return trip Vienna - Friedrichshafen, special card with the route of the Austrian Flight drawn on it, from May 2, 1929, with a mixed franking of Czechoslovakia / Austria, delivered from Prague on July 11th with airmail 1 and 2 K, by airmail to Vienna, where 2 S Crane were cancelled and an Austrian special stamp for the Austrian Flight was added, with the final destination being Lindau. Very rare, only 3 examples known so far! Photo certificate Foerster SBPV