The 18-cent Leaving Panama for San Francisco regular stamp (Scott 145) was first sold to the public on June 1, 1949. It is the high value of a four-stamp series marking the centennial of the California Gold Rush and the part Panama played in that historic era. The design features a representation of the side-wheel steamer/sailing vessel the S.S. Panama of the Panama Mail Steamship Line, which plied the Pacific Ocean between San Francisco and Panama during that period. The ship is just getting upstream for an outbound trip, and the City of Panama is in the right background. In the left background are the islands in the Bay of Panama, where these ships actually anchored because the reefs and the extreme tidal fluctuations (in excess of twenty feet) made it impossible to actually pull-up to the shore.
The 18-cent Gold Rush Centennial stamp had a number of possible uses if used singly—for example, used when the minimum registration fee was fifteen cents and ship mail was three cents or when a triple-weight airmail letter to the United States cost eighteen cents, as did five- to six-ounce seamail letters. Since these usages were not often necessary, the stamp usually appears in combination with other stamps in a 'make-up' rate scenario.
The number of these stamps shipped to the Canal Zone numbered 525,000. This was 25,000 more than any of the other stamps, yet they were all sold for use on mail and to collectors. For example, nearly 33,000 were sold on the first day of issue, most of which were put on first day covers at any of the Canal Zone post offices open at that time. The 18-cent stamp is not often seen on non-philatelic mail, but relatively large numbers of them are seen on packets of stamps sent from the Canal Zone Philatelic Agency to American collectors. Cacheted first day covers by a fairly large number of artist/producers from both the Isthmus and the United States are common, and they usually display all four of the Gold Rush Centennial stamps. Such covers from the smaller townsites and military bases are much scarcer than those from the major towns and the Philatelic Agency. Its most common use was in combination with other stamps, none of which are often seen.