Just after the close of World War II, the U.S. government decided to stop paying our soldiers overseas with U.S. currency, and start paying them with specially designed Military Payment Certificates.
These notes had the full value of U.S. currency, but were only valid in U.S. military installations and exchanges. In this way, the U.S. military could control the amount of U.S. currency entering foreign hands and limit the black market in illicit goods, as only U.S. military personnel could exchange the MPC's (as they are known) into U.S. currency.
Several different series were issued between 1946 and 1973, and were used in all major military based and in our nation's many conflicts. In many cases, few were saved by returning soldiers, making them very scarce and desirable today.