The Fourteen-cent Prexie
Double-rate airmail use
Domestic airmail cost seven cents an ounce from August 1, 1958 to January 7, 1963.
This cover was sent at double the airmail rate in 1961. While production of the fourteen-cent Prexie stopped in 1957, there were no fourteen-cent Liberty or airmail stamps issued, so any correct-rate Prexie left around would be handy for the purpose.