[Alle]luya I[n conspectu angelorum

Alleluia In conspectu angelorum identified thanks to proximity to Off. Stetit angelus for the feast of Michael and by melodic comparison among all possible alleluias for that feast. 
Also the remaining upper part of the initial letter of the alleluia verse can be inferred to be the stylized majuscule 'i' of 'In conspectu' by comparing it with the 'i' of 'Introitus' in the rubric in the lower part of the main folio recto (the two folios pertain to the same manuscript).