Information on differentia
In the original Cantus index, differentiae were identified by arbitrarily-assigned numbers within each mode. There are two or three different sets of differentiae within this manuscript, indicated by lower-case letters in the "Extra" field, with some from the original layer of the manuscript, and other notated on small marginal staves.
Downey, Charles. Inventory of Utrecht, Universiteitsbibliotheek, Ms. 406 (shelfmark 3 J 7). In: Cantus Manuscript Database for Latin Ecclesiastical Chant: Inventories of Chant Sources. Directed by Debra Lacoste (2011-), Terence Bailey (1997-2010), and Ruth Steiner (1987-1996). Web developer, Jan Kolácek (2011-).
Images: Antiphonale, Hs. 406 (Hs 3 J 7), Utrecht University