An Old Master painting that was bought for just $311 earlier this year fetched $765,000 at auction this weekend. The portrait of the 16th-Century Pope Clement VII by Italian Renaissance painter Sebastiano del Piombo was sold to an anonymous buyer bidding by telephone, a Christie's statement said. The life-size portrait, painted on a slab of slate in about 1532, had been sold by rival auctioneers Sotheby's at a sale in the central English town of Chester in June when it was given a catalogue value of $549. It was later brought to Christie's where an expert identified it as the work of Sebastiano. The Financial Times newspaper quoted Sotheby's as saying that it had mistakenly catalogued the painting as a 19th-Century copy and would offer compensation to the first seller, an unidentified art dealer.