Benjamin Franklin 1868 1-cent Z-grill Stamp is probably one of the most valuable stamps in the world. In 1998 Oct in an auction one of these stamps were sold for $935,000 million USD. In 2005 person who bought it exchange it for a Inverted Jenney Stamp block which was auctioned 2005 for 2.9 million USD.
This blue stamp depicting Benjamin Franklin was issued in 1868 and it’s named because an experimental security grill, pressed into the back. There are only 2 known copies of Z-grill out there and other one is at New York Public Library with the collection of Benjamin K. Miller, who died in 1928.