Auxilium meum a domino

Source: 
Aachen (Aix-la-Chapelle), Domarchiv, G 20
Folio: 
054r
Sequence: 
17
Feast: 
Fer. 2 per annum
Office/Mass: 
V2
Genre: 
A
Position: 
5
Cantus ID: 
001536
Mode: 
3?
Finalis: 
E
Differentia: 
C1
Extra: 
043r
Indexing notes: 
differentia is "F-F-E-D-F-F"; is it placed incorrectly on the staff? (this appears to be the shape of 3.C1 but down a third);