German-English Luftschiff Expedition to New Guinea
In 1913, Germany and England began planning a joint expedition to survey New Guinea by zeppelin. In order to promote and help finance the expedition, a series of stamps was printed beginning in July 1913. These stamps are considered cinderellas, as they were not valid for postage.
The Luftschiff Expedition was scheduled for 1914, but was canceled due to World War I. The stamps never made it into widespread circulation, and are considered rare.