The 1-dollar airmail stamp (Scott C20) is the high value of this series commemorating the 10th anniversary of airmail service in the Canal Zone and the 25th anniversary of the opening of the Panama Canal to world shipping. The stamp replicates a scene in which a Pan American clipper flying boat approaches a field for landing. The field must have been at Cristobal on the Atlantic side since that field was the terminal for clipper flights from the United States.
Collectors commonly refer to this stamp as 'The One Dollar Anniversary Airmail'.
Though 105,000 of the 1-dollar airmail stamp were printed, only approximately 40,000 were ever sold. Withdrawn from sale on February 28, 1941, authorities burned the majority of these now-valuable stamps on April 12.
This stamp is very rare properly used on a non-philatelic letter and is nearly always seen in combination with the other stamps of this series on first day covers.