The Z-Grill (Best known Stamp)

The Benjamin Franklin Z-Grill is a 1-cent postage stamp issued by the United States Postal Service in 1868 depicting Benjamin Franklin. With the embossed "Z-Grill" "waffle" on the back, this is considered the rarest and most valuable US stamp. A "Z-Grill" is a specific pattern of tiny squares embossed into the paper and visible on the back of the stamps.
The purpose of the grills was to permit the canceling ink to be absorbed into the stamp paper, thus preventing those who wanted to from cheating the postal service by washing out cancellation marks. The use of grills was not found to be practical and they were soon discontinued.

This 1868 1 cent "Z-Grill" stamp sold for $935,000 in 1998 to Mystic Stamp Company, a stamp dealer. Siegel Auctions auctioned the stamp as part of the Robert Zoellner collection. Zachary Sundman, the eleven-year-old son of Mystic Stamp Company President Donald Sundman, was the individual responsible for wielding the paddle and doing the actual bidding.

Later, in late October 2005, Sundman traded this Z Grill to financier Bill Gross for a block of four Inverted Jenny stamps worth nearly $3 million. By completing this trade Gross became the owner of the only complete collection of U.S. 19th century stamps.

Both the Z Grills were on display at the National Postal Museum along with the first part of the Benjamin Miller Collection from 27 May 2006 till 1 October 2007.