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.
Speculations about Mona Lisa
The same claim has been made for a version in the Vernon collection. The Vernon "Mona Lisa" is particularly interesting because it was originally part of the collection at the Louvre. Another version, dating from c. 1616, was given in c. 1790 to Joshua Reynolds by the Duke of Leeds in exchange for a Reynolds self-portrait. Reynolds thought it to be the real painting and the French one a copy, which has now been disproved. It is, however, useful in that it was copied when the original's colors were far brighter than they are now, and so it gives some sense of the original's appearance 'as new'. It is in a private collection, but was exhibited in 2006 at the Dulwich Picture Gallery.