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)
Although there were elements that clearly distinguished the replica from the Louvre's version, the most striking point was the fact that until the restoration of the Prado's version, no landscape background was visible. That landscape was fully recovered through the restoration carried out by the Prado between 2010 and 2012, for a request to include the Prado's work in a temporary exhibition in the Louvre called "Leonardo's last masterpiece: The Sainte Anne", from 29 March to 25 June 2012. The oxidized varnishes indicated that the black repaint had been made 200 years after the copy was painted, that is to say, not before 1750. During this restoration process, the copy was also submitted to a study of infrared reflectography and radiography, where the results revealed that the painting had been made in the workshop of Leonardo da Vinci at the same time as the original "Mona Lisa" was painted. The "key" of this discovery was the underlying drawing, as it is the same but with different style in both paintings, as well as the drawing's corrections.