This is a poster stamp version of the map that was a significant factor in the fight for suffrage in America. For details on the origin of the suffrage map and its critical importance in gaining the vote for women, see Notes for ID #1193, "Votes for Women a Success." For other examples of the suffrage map, Search > "Suffrage."
In philately, anything that looks like a postage stamp, but isn't intended or allowed for that purpose, is referred to as a "cinderella" stamp (after the beautiful but initially underappreciated folk heroine). "Poster stamps" are cinderella stamps that are copies of or resemble posters; they often carry promotional or political messages. Although they are still widely collected, the popularity of poster stamps peaked at about the time of this one, issued in 1915.
For another poster stamp issued at about the same time, see ID #2593, "Taisez-vous! La Pieuvre Germanique a des tentacules partout. [Be quiet! The German Octopus has tentacles everywhere.]" (1917).
