The first commemoratives of the twentieth century were also the first bi-colored postage stamps printed by the Bureau of Engraving and Printing. Six stamps in denominations of 1-cent, 2-cent, 4-cent, 8-cent and 10-cent were issued to publicize the 1901 Pan-American Exposition held in Buffalo, New York.
The stamps depict modes of modern transportation. All the stamps bear the inscription “Commemorative Series 1901.”
Three of the Pan-American Exposition stamps exist as inverts: the 1-cent, 2-cent, and 4-cent. The 1-cent and 2-cent were produced by accident and discovered by the public. The 4-cent invert was created intentionally after it was erroneously reported that the public had discovered inverts of this denomination. These three classic inverts were reproduced one hundred years later on a 2001 souvenir sheet.
Encyclopedia of United States Stamps and Stamp Collecting
May 16, 2006