A 50-cent carmine airmail stamp showing a head-on view of the Martin M-130 China Clipper was issued on February 15, 1937, in Washington, D.C., to pay postage on mail carried on the expanded transpacific air mail route. A 20-cent stamp (C21) of the same design was also issued on that date. Both replicated the design of the earlier China Clipper airmail stamp (C20) except that the inscription "NOVEMBER 1935" was deleted.
The expanded transpacific route now included Macao and Hong Kong in addition to San Francisco, Honolulu, Midway, Wake, and Guam. Rates were listed in the USPOD announcement dated March 9, 1937, and varied from twenty cents for the U.S. mainland to Honolulu for twenty cents, and up to seventy cents to Hong Kong. The two issues of 20-cent and 50-cent could be used to make up the new rates of twenty, forty, fifty, and seventy cents. While the two new stamps were issued for use on the transpacific service, they were valid for all airmail purposes.