Today in Postal History

This card was flown on a demonstration flight during the Rhein and Main Air Mail Post Card Week.
(The 'week' lasted from June 10 to June 23.)
Flights were made between Darmstadt, Frankfurt, Mainz, Offenbach,
and Worms by the Zeppelin Schwaben and the airplane Gelber Hund (Yellow Dog).

This card was posted in Darmstadt and postmarked with
three special CDS inscribed Flugpost am Rhein u. am Main.
The destination was Mainz.
Darmstadt is southwest of Frankfurt am Main and Mainz is south of Frankfurt am Main.
The two cities are about 30 km apart.

The cards were available in yellow (common) as well as red (scarce).

The card is franked with two of the Gelber Hund surcharged semi-offical stamps.
The overprinting consisted of a diagonal Gelber Hund,
1 M. at the top, and bars across the previous 10 pf. denomination.
The Schwaben link above tells us that the stamps were printed
for the Grand Duchess of Hesse who sponsored the event for charity.
Scott does not list these issues.

The card is also franked with a 5 pf. green Germania issued in 1905 (Scott 82).