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.

The Treskilling (Best known Stamp)

The "Treskilling" Yellow, or 3 skilling banco error of color.

postage stamp of Sweden, and the most valuable stamp in the world.
In 1855, Sweden issued its first postage stamps, a set of five depicting the Swedish coat of arms, with denominations ranging from 3 to 24 skillings banco. The 3-skilling banco value was normally printed in a blue-green color, while the 8-skilling was printed in a yellowish orange shade. It is not known exactly what went wrong, but the most likely explanation is that a cliché of the 8-skilling printing plate (which consisted of 100 clichés assembled into a 10 x 10 array) was damaged or broken, and mistakenly replaced with a 3-skilling cliché. The number of stamps printed in the wrong color is unknown.

After changing hands several times, Sigmund Friedl sold it to Philipp von Ferrary in 1894, who had at that time the largest known stamp collection in the world, and paid the breathtaking sum of 4,000 gulden. As time passed, and no other "yellows" surfaced despite energetic searching, it became clear that the stamp was not only rare, but quite possibly the only surviving example.

When Ferrary's collection was auctioned in the 1920s, Swedish Baron Eric Leijonhufvud acquired the Yellow, then Claes A. Tamm bought it in 1926 for £1,500 (GBP) in order to complete his collection of Sweden. In 1937, King Carol II of Romania purchased it from London auction house H. R. Harmer for £5,000, and in 1950 it went to Rene Berlingen for an unknown sum.

In the 1970s, the Swedish Postal Museum caused controversy by declaring the stamp to be a forgery but after examination by two different commissions, it was agreed that it was a genuine error.

In 1984 the stamp made headlines when it was sold by David Feldman for 977,500 Swiss francs. A 1990 sale realized over one million US dollars, then in 1996 it sold again for 2,500,000 Swiss francs. Each time it has been sold it has set world records.