A stamp that initially appeared to be a rare 1918 "Inverted Jenny," used by a Florida voter to mail an absentee ballot last week, is probably a fake, an official with the American Philatelic Society said on Tuesday.

After viewing photographs of the stamp, which turned up on a ballot envelope in Fort Lauderdale, officials with the U.S. stamp collectors' organization said they found inconsistencies that led them to believe it was a reproduction.

The Inverted Jenny took its name from an image of a biplane accidentally printed upside down. Only 100 of the misprinted stamps have ever been found, making them among the rarest in the world of philately.