Swiss Pro Aero stamp was issued in 1938 to promote the Pro Aero Campaign, held throughout Switzerland from May 21 through May 22, 1938. This day special Air Mail flights were made to thirteen Swiss cities, with the letters being franked by Swiss Pro Aero stamp. The interesting fact is that the unused copies were not sold separately; they were affixed to Air Mail letters by the postal clerks. That is why the mint examples are difficult to find. Such a scarce version of Swiss Pro Aero stamp in mint never hinged condition, with a good colour and strong centering, was sold for $9,000 by US philatelic auction house in December, 2012.