20-cent Airplane National Postal Museum

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