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.
Isleworth Mona Lisa
Also, according to Henry F. Pulitzer in his book "Where is the Mona Lisa?" (1960), Gian Paolo Lomazzo, an artist and writer, refers in his "Trattato dell'arte della Pittura Scultura ed Architettura" (1584), to ""della Gioconda, e di Mona Lisa" (the Gioconda, and the Mona Lisa)". "La Gioconda" is sometimes used as an alternative title of the "Mona Lisa" hanging in the Louvre; the reference implies that these were, in fact, two separate paintings. Pulitzer reproduces the critical page from Lomazzo's tract in his own book.