The 40-cent Gaillard Cut airmail stamp (Scott C-13) was one of the originals in this series offered for sale to the public on November 18, 1931, and some 826,00 were printed through the years. At the time there was almost no call for it as a single stamp, so its use was almost always in combination with other stamps. In its lifetime through the 1937 and 1940s rate changes, it is uncommon in any form on cover. Perhaps most often it is seen on letters addressed to Latin American destinations such as Brazil and Argentina.
Between 1941 and 1951, some 30,500 of these stamps were overprinted for official use by Canal Zone-based federal offices. There are two types of overprints on this stamp known as 'The Official Overprints' (Scott CO6 and CO12).
Since most people could not afford the cost of the full series on cover, the first day covers of this stamp are very scarce. Those that do exist are almost always found with the other value, which makes for a very attractive cover.