Orange County billionaire Bill Gross, one of the world’s premier connoisseurs of rare stamps, is selling a major part of his $42.5 million U.S. collection and giving the proceeds to charity.

Gross’ philanthropy is well known, but not so much his reputation as a philatelist.

The man who built his fortune as the bond baron of the business world has a stamp gallery at the Smithsonian and a discerning eye for adhesive-backed treasures. He is one of a relative few who can appreciate an “inverted Jenny” or a “1-cent Z Grill” — both part of a 2005 stamp swap that made international philately history.