US Canal Zone Airmail Postage, Plate Block proof<br>The 25-cent Airmail stamp (Scott# C-18)<br>The series honored the 10th anniversary of airmail service in the Canal Zone along with the 25th anniversary of the Panama Canal opening.<br>200,000 copies of the 25-cent airmail were shipped to the Isthmus, but only a little over 82,000 were actually sold. The unsold remaining stamps were destroyed on April 12, 1941.
U.S. Government, Canal Zone Administration
1939-07-15
Smithsonian Stamp museum