I seem to remember we discussed the following question before -- some time the last twenty years! -- but I can't find it.

What is known about the history of the Mass Ordinaries as formulated in the Liber: Masses I to XVIII, with their titles and assignments? Did the Liber’s first editors invent them? Or extend some previously known formulations? Are there earlier or much-earlier assignments of particular chants to particular feasts? (I suspect Mass I Kyrie for Easter, at least.)