Mr. Gross's collection includes a full sheet of stamps from 1851.
Some of the rarest and most valuable American and foreign postage stamps are being displayed in a once-in-a-decade event, the Washington 2006 World Philatelic Exhibition.
Among the diminutive paper gems is a 1-cent blue stamp from 1868 with a rare experimental security impression that collectors call the "Z-grill," only two of which are known.
The Z-grill can be seen as part of the collection of William H. Gross, the well-known bond investor and managing director of the Pimco firm. His collection is the only one to contain every United States stamp issued in the 19th century.
Mr. Gross said in a telephone interview on Friday that he collected stamps as a child and resumed the hobby in the early 1990's as a way to "reconnect with my childhood." He said he was determined not to "get clipped"; his mother had once tried to invest by buying sheets of new stamps from the post office, only to find later that they were worth nothing more than their face value.
Mr. Gross, 62, said he decided to bring his investment experience to bear and researched the historical trends in the auction prices of rare older stamps before immersing himself in the hobby again. He found that over the long term, scarce and high-quality specimens appreciated at least as well as the economy in general and provided a sound way to put serious money into collectibles.
At first, Mr. Gross pursued collecting out of the public eye, exhibiting his stamps under the pseudonym Monte Carlo at national philatelic gatherings. He later went public and continued to develop his collection under his own name. In a highly publicized auction last fall, Mr. Gross bought a block of four rare misprinted airmail stamps for nearly $3 million and traded them for the Z-grill, completing his 19th-century collection.
Mr. Gross has decided to continue supporting the hobby by lending his Z-grill to the Smithsonian National Postal Museum after the exhibition closes. There, it will be displayed alongside the only other known copy, part of the Benjamin K. Miller collection belonging to the New York Public Library.
The exhibition at the Washington Convention Center is free and will feature special events. The organizers are expecting as many as 100,000 people to attend before the show closes next Saturday, and they say the total value of the stamps on display exceeds $200 million.
