But for the co-founder of Pimco, who is now a fixed-income portfolio manager at Janus, this is not an average stamp collection and Gross is not an average stamp collector. In October, Gross will begin selling his U.S. stamp collection, a treasure trove that Cheryl Ganz, former chief curator of the Smithsonian National Postal Museum in Washington, D.C., calls the ”greatest U.S. stamp collection ever.”

Gross, 74,  is only the second person to have ever formed a complete collection of 19th century postage stamps, and his collection, valued at roughly US$40 million, is the “largest and most valuable private collection of U.S. stamps in the world,” according to Siegel Auction Galleries.

Siegel will be offering the stamps for sale during at least four sales beginning with an auction of the finest pieces on Oct. 3 at the New York Palace Hotel.

Ganz was speaking at a press event Tuesday revealing details of the first sale, including a “2¢ Blue Hawaiian Missionary,” estimated between US$500,000 and US$750,000 and a “unique block of the 24¢ 1869 Pictorial Inverted Center,” with an estimated value between US$750,000 and US1 million. The “block,” which means a grouping of attached stamps, in this case of four stamps, includes an engraving of an image of John Trumbull’s Declaration of Independence that was inverted by mistake.

While high-ticket items abound in the sale, there’s a “boatload of items” in the US$50,000 to US$100,000, says Charles Shreve, a director at Siegel and Gross’ close advisor on his collection for some 25 years.

Gross began collecting stamps in 1993, employing a philosophy from the get-go to “hit it hard, make acquisitions,” whenever a big collection came to market, Shreve says, a strategy consistent with his approach to the bond market. But Gross also became a serious student of stamps, leaning from the outset on Shreve’s expertise. Gross’ philately, as the field is known, led him to donate US$10 million to the National Postal Museum in 2013, leading to a 12,000-square-foot expansion, called the William H. Gross Stamp Gallery.

Unlike art, where many of the most valuable pieces are in museums, most of the best collectible stamps are in private hands. Gross’ plan to sell his collection is intended to put these valuable stamps back into market circulation and to attract new collectors to the field.