Having seen this a number of times and hearing different guides, as I recall he never stopped working on this piece, it was even beside his bed as he got old and was dying it was still a work in progress.
I remember reading that she had a veil on her head (a Mantilla?) at one point in his painting of her, and the ‘shadow-marks’ on her forehead were from the lace that he later painted out.
Funny the things that pop into your head about these important pieces of art, right or wrong. ;)