New air mail postage rage brought need for this design, of the smaller size for
stamp rolls ("coils"), vending machines and booklet.

The Douglas DC-4 Skymaster was developed during World War II and led to the DC
-6 and DC-7 series with pressurized cabins. The rapid development of aircraft
was brought about by U.S. dominance of air transportation during and after
World War II.  In 1951 the United States built more than 80 percent of the
world's airliners.