The Seven-cent Prexie
Domestic air mail use
A seven-cent Prexie was used to pay the domestic air mail rate in effect from August 1, 1958 through December 31, 1962. Production of these stamps had ended in 1957, but there were supplies of them in various post offices.
Postage rates for APO's were the same as those for the U.S. A seven-cent prexie was used on this cover to pay the airmail rate from Kunsan, Korea to California in 1961. The seven-cent jet airliner stamp was available there as well from the evidence of another cover I've seen, so the APO may have been trying to use up a supply of Prexies where it could.