The four stamps [bond investor Bill Gross] is trading today are a block of the famous 24-cent airmail stamps from 1918 that are known to collectors as the Inverted Jenny because the Curtiss JN-4 biplane depicted in the center was accidentally printed upside down.
Though some other Inverted Jenny stamps exist, Mr. Gross's block - known as the Inverted Jenny plate block - is the only one showing a marginal printing plate number, also produced inverted. He bought the block anonymously at auction on Oct. 19 for $2.97 million (including the 10 percent buyer's premium).
The prize for which he is trading it is a rare 1-cent Z-grill, a blue stamp depicting Benjamin Franklin that was issued in 1868 and is so named because of an experimental security grill, pressed into the back, whose purpose was to bar reuse by keeping the cancellation from being washed off. It belongs to Donald Sundman, president of the Mystic Stamp Company of Camden, N.Y.
"The Z-grill is like the Hope Diamond of American philately," said Mr. Sundman, whose company sells stamps to beginners and advanced collectors alike.
"This trade is the most exciting thing to happen in the hobby in over a decade," said Charles Shreve, the stamp dealer and auctioneer who arranged it.
Mr Shreve added, "It narrowly beats out the time in 2000 when Juan and Marcos of Buenos Aires came this close to selling a complete block of defective Weimar Republic queens."