"Thank you very much, Mr. Milanese, let's see if it fits other circumstances.
Jan C. Guido Milanese said:
For the first item (actus nostros dirige) I find some evidence of a possibly trad. Tyrolean hymn (or prosula, maybe), used apparently by the…"

"Thank you very much! That would be also a plausible explanation.
Jan Jeffrey Dean said:
It was customary throughout the middle ages for books to be inventoried with the incipit of their SECOND folio, chiefly to distinguish different books that…"

"For the first item (actus nostros dirige) I find some evidence of a possibly trad. Tyrolean hymn (or prosula, maybe), used apparently by the Redemptorists: see https://list.uiowa.edu/scripts/wa.exe?A2=piporg-L;4968d208.0409B (Tyrolean) and a…"

"It was customary throughout the middle ages for books to be inventoried with the incipit of their SECOND folio, chiefly to distinguish different books that began the same (or whose contents were altogether the same), but partly in case the first…"

"Sorry, my fault. It was the sequence (also in D-Mbs cgm 716). I didn't see at first that you already identified it :-) Terrasa Xavier a dit :
a Minnesänger song ? Which one ?"