On September 15, 1997, the USPS issued a stamp whose vignette consisted entirely of microprinting. The experimental printing technique was used to discourage and protect against counterfeiting.
US #523, the 1918 $2 orange-red Franklin error stamp, was first used on or around August 19, 1918. However, it would be two years before the error was discovered.
On December 1, 1921, the US Post Office Department opened its Philatelic Agency in Washington, DC, to the benefit of stamp collectors.
On May 9, 1939, the Philatelic Truck departed the White House on a cross-country journey to introduce thousands of Americans to the exciting world of stamps.