The USPOD Philatelic Agency
On December 1, 1921, the US Post Office Department opened its Philatelic Agency in Washington, DC, to the benefit of stamp collectors.
By the 1920s, stamp collecting had grown to be very popular both in the US and around the world. In spite of that popularity, the number of post offices in the country had dropped from over 76,000 in 1900 to about 52,000 in 1920. Collectors and dealers began writing directly to the Post Office Department to request harder to find stamps.