Hi. According to old letterfolds, here http://www.metmuseum.org/toah/works-of-art/19.73.120 there is Durer painting of Erasmus of Rotterdam, (1526). A letter folded can be seen there.
But in LEONARDO da Vinci, Portrait of a Musician, 1490, http://arthistory.about.com/od/leonardo/ig/leonardo_paintings/ldvpg_10.htm the musician has a paper in his hands with some interesting folds Good luck, Jose Tomas Buitrago
