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.
David Feldman (philatelist)
In 2004 Feldman was mandated by Elizabeth Meyer to make research, scientific and physical examinations of her painting which had been known as the Isleworth Mona Lisa. In 2010, he formed, together with Markus Frey and Daniel Kohler, The Mona Lisa Foundation, a non-profit organisation, which absorbed decades of previous research concerning that painting, and initiated new expertises, scientific tests and physical examinations. The work culminated in the publication of 'Mona Lisa – Leonardo's Earlier Version', a 320-page volume published in 2012. The exact status of the painting, however, continues to be debated by art historians.