The Six-cent Prexie
Sheet stamps were first released on July 28, 1938. A total of 3,614,501,500 were issued from then through 1956. Horizontal coil stamps were issued from 1939 through 1960. Total production was 476,283,000.
The primary use of the six-cent Adams was to pay the domestic air mail rate in effect from 1934 into early 1944, and again from 1949 until mid 1958. This rate was applicable to air mail to Canada during the first period, and Canada and Mexico during the second. It was also the military air mail rate to and from U.S. Armed Forces personnel stationed outside the continental United States from Christmas of 1941 until late 1946.
The six-cent military concession air mail rate was extended to contractors and civilian employees using Army or Navy post offices from late 1942 until late 1946. A special six-cent air mail rate was allowed for letters sent to prisoners of war and the like held in foreign countries from early 1944 until late 1946 as well.
Several six-cent combination rates were also possible during the period. These include combination of air mail in the US and surface mail to PUAS countries or to or from Hawaii, and post cards to UPU countries with air mail service in the US only. A surface letter deposited at a dispatching post office after the mails were closed was charged a three-cent late fee to be put on a particular ship to or from a U.S. possession such as Hawaii from 1932 until 1958, making the total postage six cents. During this same period a letter mailed aboard ship within the U.S. was subject to doubling of the three-cent first-class rate.
Six cents was also the minimum charge per piece for a controlled circulation publication from early 1944 through the end of 1948. It is also possible to find a Prexie paying the six-cent air mail post card rate in effect from 1963 through 1968, although that is very late usage.