On the initiative of Grand Duchess Eleonore von Hessen und bei Rhein, the “Rhine-Main Airmail Days” took place from June 10 to 23, 1912. The Zeppelin „L.Z. Schwaben“ and the Euler biplane “Gelber Hund” (Yellow Hound) transported mail between the cities of Darmstadt, Frankfurt, Mainz, Offenbach and Worms. A special private postcard was initiated and issued by the Grand Duchess for the event. The postage for the flights was paid with semi-official airmail stamps. The subsequent transportation to the recipient was paid for with the regular stamps of the German Empire (Germania stamps at the time).

In addition, a privately produced but officially approved special cancellation was used. On June 14, 1912, the Reichspostministerium (Reich Postal Administration) intervened and prohibited the use of the special cancellation for the airmail stamps, demanding that they be left uncancelled. The Grand Duchess immediately and successfully objected to this, so that on June 16, 1912 the practice of using the special cancellations for the airmail stamps was resumed. Due to the brief intervention, there are covers on which the airmail stamps were not canceled on June 15, 1912.

There are also postcards on which the airmail stamp(s) are canceled with the special postmark dated June 16, 1912, while the regular stamps bear the same postmark dated June 15, 1912. However, such postcards are rarer in practice than postcards bearing both kind of stamps with the special postmark dated June 15, 1912. Apparently either not much attention was paid to the prohibition on the day of its “validity” or the airmail stamps with the date June 15, 1912 were simply canceled retrospectively.