The 5-cent stamp (Scott C7) of this series was issued on November 18, 1931, as part of the original group. It was used exclusively as a 'make-up' rate stamp until October 31, 1946, when the regular first-class airmail rate to the United States was reduced to that figure. This period of use ended when the rates increased to six cents, effective January 1, 1949, after which time the 5-cent stamp reverted to its former multiple usages.
Some 51,000 of these 5-cent airmail stamps were overprinted for official use by Canal Zone-located federal offices between 1941 and 1951. There are two types of overprints on this stamp known as 'The Official Airmails' (Scott CO1 and CO8).
The stamp and covers using it are readily available because just over 10,000,000 were printed over time. Interestingly, it is well known on first day covers prepared by one local individual but much less so in combination with the other stamps of the initial group. It is also fairly frequently seen in combination with 1-cent stamps on servicemen's letters during World War II as they enjoyed a special preferential 6-cent airmail rate at that time.