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.
Mona Lisa (Prado's version)
Previous to its restoration, the painting was catalogued as an anonymous copy from the first quarter of the 16th century. However, it usually hung in the Prado alongside masterpieces of Italian artists such as Raphael or Andrea del Sarto; a lithography of the painting also exists, which is unusual for an anonymous copy.