 In 1921, postal officials closed the College Park airmail station to focus on routes where airmail was clearly superior in speed and cost to the railroad.However, the field remained home to researchers, inventors, and businesses focused on developing commercial aviation.The original Air Mail letter rate per ounce between any two points on the route when service began was 24 cents per ounce for which the first special purpose U.S. Air Mail stamp (C-3) was issued on May 13, 1918.