Isleworth Mona Lisa
According to Leonardo's early biographer Giorgio Vasari, Leonardo had started to paint "Mona Lisa" in 1503, but "left it unfinished". However, a fully finished painting of a "certain Florentine lady" surfaced again in 1517, shortly before Leonardo's death and in his private possession. The latter painting almost certainly is the one that now hangs in the Louvre. Based on this contradiction, supporters of the authenticity of the "Isleworth Mona Lisa" claim it is the unfinished "Mona Lisa", made at least partially by Leonardo, and the Louvre "Mona Lisa" a later version of it, made by Leonardo for his own use.
Hugh Blaker
Shortly before World War I, Hugh Blaker discovered a painting of the "Mona Lisa" in the home of a Somerset nobleman in whose family it had been for nearly 100 years. This discovery led to the conjecture that Leonardo painted two portraits of Lisa del Giocondo: the famous one in The Louvre, and the one discovered by Blaker, who bought the painting and took it to his studio in Isleworth, London, from which it takes its name. According to "Encyclopedia Americana" and "The New York Times", The Isleworth Mona Lisa has been attributed to Leonardo, and is thought to be the unfinished portrait from which Raphael made his famous sketch (which is in the Louvre museum), and is in no sense a copy of the Louvre "Mona Lisa".