I know very little about Flugpost (Airmail) prior to official airmail service in Germany beginning in 1919. PostmasterGS has written and updated posts about early Zeppelin mail, but I haven't read anything about the Flugpost am Rhein und Main issues of 1912.
I purchased a flown cover at auction this past weekend, so decided to do a little research.
There is a lot of information available online about the "Flugpost am Rhein und Main" semi-official stamps, but I could only found it in German.
A rough translation of the website
http://www.flugpost-rhein-main.de/ gives a history of the service, but the Google translator is in need of some help.
Here is what I've gleaned so far.
1) Service took place for a very short period from 10-23 June, 1912 as part of an charity event called "Postcard Week". The proceeds went to the "Grand Ducal Office" for mother & infant care in Hesse.
2) Two airships were used: the Schwaben and Gelber Hund (Yellow Dog). The Schwaben, a Zeppelin, carried the bulk of the mail, while the Gelber Hund, an airplane, carried lesser amounts of mail. Flown covers on the Gelber Hund are uncommon.
A quick Google search turned up the following images of the GelberHund, Schwaben, and a flown cover (with Gelber Hund overprinted stamp) ...


The airship "Schwaben" ...

And finally, a photo of the auction lot I purchased. The seller also had a Gelber Hund flown cover that, had I known it's scarcity at the time, I would have tried to acquire it. It sold for $115. This lot, with a nearly perfectly centered flugpost stamp on cover and a set of 3 unused stamps to boot was purchased for $35. Both are bargains.
Any additional comments, information, or links are welcome!
Brian
