Many airmail postcards and envelopes document specialty flights that were not from routes authorized by the government. These flights include both official mail authorized by the Post Office Department for special flights and unofficial mail carried privately and postmarked before or after the flight. Specialty flights take place at unique events such as air races, by various types of aircraft such as jets, or related to famous aviators such as Charles Lindbergh. Collectors refer to these as “souvenir historical covers” because they often document historical moments in aviation history.
Cheryl R. Ganz, National Postal Museum