C9 - 1927 20c Airmail Perf 11 - Mystic Stamp Company
College Park Aviation Museum - Park and Recreation - Prince Georges County MD
Air Stamps–Heavier than Air: Biplanes, Monoplanes, Jets & Helicopters
NATIONAL POSTAL MUSEUM - 825 Photos & 197 Reviews - 2 Massachusetts Ave NE, Washington, District of Columbia - Museums - Phone Number - Yelp
Chronology: 1980-1989 Air & Space Forces Magazine
The Postal Airplanes National Postal Museum
China Clipper - Wikipedia
Will an Inverted Jenny Turn the Stamp World Upside Down Again?
The Postal Airplanes National Postal Museum
National Postal Museum Smithsonian Institution Archives