NEW YORK (AP) – It was a blockbuster trade. No, not in sports. In multimillion dollar stamps.

A block of four stamps from 1918 bearing the image of an upside-down plane – valued at $3 million – were swapped Wednesday for a single stamp with an equal value of $3 million – the world’s highest-priced single stamp.

A California collector traded his four “Inverted Jenny” stamps for an 1868 one-cent “Z Grill” stamp to fill out what experts believe is the only complete collection of 19th-century U.S. stamps.

“This isn’t like kids swapping stamps,” said Charles Shreve, president of Shreves Philatelic Galleries in Manhattan, where the trade was made. “The ‘Z Grill’ is like the Holy Grail of U.S. stamps – it’s the Hope diamond of American philately.”

At the center of Wednesday’s swap are five stamps that sold for 97 cents in 1918 at a Washington, D.C. post office. At the heart of the deal is Bill Gross, of Newport Beach, Calif., known as “the bond king” for his investment success at the Pimco fund giant. Gross was missing the one-cent “Z Grill” stamp owned by Donald Sundman, president of Mystic Stamp Company of Camden, N.Y.

Two weeks ago, Gross paid almost $3 million at a New York auction – a world record price for any stamp – for the four 24-cent stamps with the mistakenly printed upside-down Curtiss JN-4 biplane, known as “Jenny,” that first delivered the country’s air mail. (In 1996, a Swedish stamp known as the Treskilling Yellow sold for $2.2 million).

The previous record for a philatelic item at auction was $935,000, which Sundman paid seven years ago for the “Z Grill” – outbidding Gross. It is valued at about $3 million today.

Gross now has “a comprehensive, complete collection representing every 19th-century U.S. postage stamp ever made,” said Shreve, whose company had placed his successful auction bid for the four stamps.

So what’s so special about a stamp for it to be worth $3 million?

Small, blue-hued and bearing the image of Benjamin Franklin, the “Z Grill” is so named because of the waffle-like grill pressed into the back that better absorbed postmarks and prevented people from washing off the cancellation and reusing the stamp.

It’s one of only two existing “Z Grill” stamps; the other one is owned by The New York Public Library.

“It’s like the Grand Canyon. Marvelous,” said Sundman, who would enjoy looking at his prized philatelic possession for 10 minutes at a time. “It’s special. It’s rare.”

Sundman plans to show his “Inverted Jenny” block at exhibitions around the country.

“Collectors find errors very exciting,” he said. “It’s the thrill of the chase – and that you might find one.”

Gross was not present Wednesday and would not immediately comment on the trade. He plans to exhibit his newly acquired “Z Grill” at the Smithsonian Institution in Washington in May, at the same time as the “Inverted Jenny” is to be displayed at the National Postal Museum, according to Shreve.

First, the stamp dealer must deliver the $3 million stamp to Gross in California.

Make no mistake, no stamp will be needed to mail this stamp. It’ll be personally carried by Shreve, who would not reveal when or how, for security reasons.

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