The wrong thickness, inexact colours and the incorrect number of perforations have exposed a stamp once believed to be a rare "Inverted Jenny" as a forgery, cancelling hopes collectors had found something worth hundreds of thousands of dollars.

Stamp expert Mercer Bristow of the American Philatelic Society examined the postage used to mail an absentee ballot for the Nov. 7th U.S. Senate and House of Representatives elections for about half an hour before identifying it as a fake.

He compared the fake at Broward County's elections office with authentic stamps and other forgeries.

The true 24-cent stamps were printed in 1918 with a First World War-era biplane erroneously flipped upside down.

Experts with the Pennsylvania-based society confirmed their initial suspicion the stamp was not authentic after previously studying a digital photograph and noting it resembled a forgery they have stored in their library.

But they couldn't rule for certain until seeing it in person.

The copied stamp was first noticed by a county commissioner and former stamp collector reviewing absentee ballots after the Nov. 7 election. The ballot the envelope contained was not counted because it did not contain the person's signature.

With files from the Associated Press