1923 Second Airmail #C4-C6

1923 Second Airmail Issues #C4-C6
With the arrival of nighttime airmail service in 1923, the U.S. was divided into three zones: New York to Chicago, Chicago to Cheyenne, Wyoming, and Cheyenne to San Francisco, with a different rate paid for each zone crossed. Previously, mail was flown during daylight and transferred to trains at night.