The US postal service started constructing airway beacons between 1923-1933 to aid navigation before there were good aviation charts for pilots. A series of lighted beacons were placed every 10 miles or so along with a brightly painted concrete arrow for navigation during inclement weather and at night. The lights would flash a code to identify each beacon's number. By 1940, when technology had improved, the beacons were torn down and the steel went towards the war effort.
for the letter A - air mail