The fourth in the Transport Plane Series, the 20-cent value was again the same design but in a bright green color. The first day of issue was on August 27, 1941, in Philadelphia, Pennsylvania. This stamp's primary purpose was to pay the twenty-cents-per-half-ounce airmail rate between Hawaii and the continental United States. This airmail route was over the Pacific Ocean between California and Hawaii and was known as FAM (Foreign Airmail Route) 14, which had been inaugurated in November, 1935.
It also paid the only two other twenty-cent rates then in effect: the inter-island rate between Guam and Hawaii and the rate from the continental U.S. to British Honduras in Central America. There were no other twenty-cent rates until April 1, 1945, when rates to several South American countries were reduced. However, the 20-cent transport issue was conveniently used in combination with other airmail or definitive stamps to pay higher international airmail rates.