The three Transpacific issues (Scott C21-C23) offer a head-on view of the Martin M-130 flying boat, the China Clipper. In the early 1930s, seaplanes became the preferred mode of transportation to the Caribbean and South Carolina for both mail and passengers. Not long afterwards, the same was true for transport across the Pacific and Atlantic oceans. Payloads steadily increased, as did the distances traversed, and these advances demanded greater fuel capacities and larger airplanes. For land planes, this meant long, hard-surfaced runways, a resource available primarily in large cities. Seaplanes needed only water to land . . . an ocean, a bay, or a lake.
Pan American Airways operated the first transpacific mail and passenger service, which linked San Francisco with Manila, with stops at Honolulu, Midway, Wake, and Guam. The China Clipper's scheduled flights began on November 22, 1935, and Capt. Edwin C. Musick oversaw a crew of seven. The flight's critical segment was the 2,400-mile span between California and Hawaii. Although the M-130 had a seating capacity of forty-one, weight restrictions limited the number of passengers to eight or less. Most often the crew outnumbered passengers.
A blue 25-cent airmail stamp (Scott C20) was issued to pay postage on mail carried on the route. The rate was based on twenty-five cents per leg—that is, a segment that had a post office at each end. With no post offices, the refueling stops at Midway and Wake didn't count as legs. On April 21, 1937, service was extended to Macao and Hong Kong. Two new stamps, a 20-cent green and a 50-cent carmine, similar to the 1935 issue but without the wording "NOVEMBER 1935," were issued on February 15, 1937. The U.S. Post Office Department listed rates in its March 9, 1937, announcement.