These additions are indicated either by the author's full name or by a simple ת (= Tosefet), and appear in MS BL 27,200–27,201, the manuscript on which all modern editions are based.
He often appends his name to such additions; and in one place he says plainly: "These explanations were added by me, Isaac b. Dorbolo; but the following is from the Machzor of R. Simchah of Vitry himself".
[3] One of his particular entries to the Machzor Vitry is of great historical value to the Jewish Community of Olomouc in Moravia, as it is the first ever certain record of Jews in that city.
"[5] An important observation sheds light on the origin of the Olomouc Jews: they almost certainly came from Western Europe, and most likely after the massacres of the First Crusade.
[6] According to Leopold Zunz,[7] Isaac's father is identical with the correspondent of Rashi and the martyr of the First Crusade of the same name; but this is chronologically impossible.