As the viewer notices several rubrical variations in the 15th century Mass, recall that this would have been according to rubrics prior to Quo Primum.
The Mass itself appears to be some kind a Missa Cantata (sung, but not solemn), possibly a monastic usage?
It would have been nice if the post included some contextual information, such as whether any particular local (e.g., Gallican) or monastic usages were incorporated into these rubrics.
Saying Mass behind the rood screen is also unusual in traditional venues today, but I wish it were more common! A very definite distinction between priest/ministers and the laity!
Note also that the choir/cantor is inside the sanctuary, as this was a LITURGICAL office: No notion of "active participation" with the faithful all getting into the act, Novus Ordo style: These faithful are silent.
Removing the chasuble during the Credo in preparation for preaching was also one I have not seen before.
If anyone can add any additional contextual information, please PM me; I don't want to derail the thread.