U.N. stamps were first issued in U.S. currency on United Nations Day, October 24, 1951. The U.N.P.A. is the only organization in the world that is not a country or a territory and yet is permitted to issue postage stamps. Further, it is the only entity to issue stamps in three currencies: U.S. dollars, Swiss francs, and Austrian shillings. The stamps generate revenue for the U.N. and provide information on U.N. achievements and activities. The stamps are valid for postage on mail from the U.N. offices in New York, Geneva, and Vienna.
The 50th Anniversary souvenir sheet has an innovative design consisting of two square stamps, each with a circular stamp inside which bears the denomination. Either the entire perforated square with the circle intact could be removed, or else just the circle stamp itself.