The 6-cent Up the Chagres River regular stamp (Scott 143) is part of the Gold Rush Centennial series issued on June 1, 1949, and made available at all Canal Zone post offices for postmarking. It depicts a small native boat called a 'bungo' ascending the Chagres to a point that it reached the head of navigation and met the old Spanish trail at the townsite of Las Cruces. This was a very difficult part of the trek, especially during the dry season, when the Chagres did not flow freely. This 6-cent stamp was meant to be used for double-weight letters addressed to the United States which were to go by boat or alone to pay the then-current airmail rate to the States.
The Bureau of Engraving and Printing produced 500,000 of these stamps, and all of them were sold.
The 6-cent Gold Rush Centennial stamp is frequently found with the other stamps of this series on cacheted first day covers by about twenty different publishers. It is found postmarked at all the then-current Canal Zone post offices. Examples on non-philatelic mail are fairly common in its intended roles, but in combination with other stamps it is surprisingly hard to find.