"who looks so like the Bellini portrait of Mahomet II"
Bellini's Mehmed II
Swann is again indulging in his habit of comparing life to art (see bookmark to p. 94, "how goes it with Giotto's charity?").
Gentile Bellini (c.1429-1507) was a Venetian painter who was sent to Constantinople to paint the Ottoman Sultan Mehmed II in 1479. The painting now hangs in the National Gallery in London.