The inscription on the letter identifies the sitter as Marco Barbarigo (about 1413 - 1486). Marco, son of Francesco Barbarigo, Procurator of San Marco (who died in August 1448), was in London from about 1445 and in February 1449 was Venetian consul in England. He became Doge in 1485. This painting is tentatively thought to have been painted during the period of his stay in London.
The National Gallery, Trafalgar Square, London WC2N 5DN