see "Material for the history of the text of the Qur'an: the old codices: the Kitab al-masahif of Ibn Abu Dawud together with a collection of the variant readings of the codices of Ibn Ma'sud, Ubai, 'Ali, Ibn 'Abbas, Anas, Abu Musa and other Qur'anic authorities which present a type of text anterior to that of the canonical text of 'Uthman." by Arthur Jeffery, Leiden: E.J. Brill, 1937.

In many cases, they were "real textual variants and not mere dialectal peculiarities as is often suggested." (Arthur Jeffery, The Qur'an as As Scripture, New York: Books for the Libraries, 1980, p97).