NEW YORK (Reuters) - When legendary bond investor Bill Gross graduated high school, his mother gave him a stack of U.S. postal stamps that she had purchased over the years and which she hoped he could sell in order to pay for college.

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Unfortunately, Gross found that the mass-produced stamps were worth less than their face value. But the gesture drove him to build one of the world’s most valuable U.S. stamp collections, valued at $42 million, part of which will be auctioned in October.

Charles Shreve, Gross’s philately adviser for more than 20 years, said Gross collected the stamps “to prove that (his) mother was right - that buying stamps can be fun and make money. She just bought the wrong ones.”

Gross, co-founder of Pimco, one of the world’s largest bond managers, is worth $2.5 billion, according to Forbes magazine. He abruptly quit the Newport Beach, California-based firm in September 2014 and soon after joined Janus Henderson Investors, where he runs the $2.1 billion Janus Henderson Global Unconstrained Bond Fund.

Items from Gross’s collection, managed by Robert A. Siegel Auction Galleries, where Shreve is the director, were on display Tuesday in New York, including a block of six stamps featuring George Washington’s portrait from 1847 that are expected to sell for as much as $750,000.

The block, called the “bible” block because it was found stuffed in a bible sometime before 1912, is just one of three remaining blocks because 5- and 10-cent stamps from 1847 were mostly thrown out after they were devalued by newer stamps the U.S. Post Office issued in 1951.

The most valuable stamps in the collection may be a set of four, 24-cent-stamps from 1869 that depict American artist John Trumbull’s painting “Declaration of Independence” upside down. The inverted stamps were part of the first batch of multi-colored stamps the Post Office printed, and could sell for $1 million.

Gross has sold part of his collection before. Between 2007 and 2014, he sold his non-U.S. stamps for a total of $27 million, money he donated to charities including Doctors Without Borders. The proceeds of this sale will also go to charity, which has yet to be named.

Shreve said Gross is selling his U.S. collection now because he accomplished a long-time goal of collecting one of every U.S. stamp ever issued.

Gross’s stamp collection will be auctioned in three or four phases starting Oct. 3.