 They remained on sale until August 11, 1952, after witch remaining stocks were destroyed.The first Canal Zone air mail stamps were issued in 1928—1929, with rates of postage over-printed on existing Canal Zone permanent issue stamps, issued in 1929.In 1938 the Canal Zone Postal Authorities issued a series of stamps commemorating the 25th Silver Jubilee anniversary of the opening of the Panama Canal, and the 10th anniversary of airmail service there.