This edition is based on Librone 1, ff. 172v–173r, where it was written by Scribe A, who copied it without mistakes. The variant spelling of x as s (‘genitris’ for genitrix in C and T) is typical of medieval Latin.
This second motet of Compère’s cycle Hodie nobis de virgine was sung to or instead of the Gloria, as the loco rubric indicates (‘loco Gloria’), but in contrast to that chant of the Ordinary, strongly focused on Christ, this motet speaks about the Virgin Mary. This change of subject (from Christ to Mary) is not surprising in a motet cycle to be sung during the Christmas season, due to Mary’s central role in Christ’s nativity: in fact, Marian materials are often found in the Christmas liturgy.^[1]
The texts utilized by Compère for his motet are clearly of liturgical origin: the first half (‘Beata dei genitrix Maria, cuius viscera intacta permanent, hodie genuit salvatorem saeculi’) corresponds to the first responsory of Matins at Christmas, and the second (‘Beata, quae credidit, quoniam perfecta sunt omnia, quae dicta sunt ei a domino’), taken from the Gospel of Luke (1: 45), is very often found as verse to the same responsory (Cantus ID 006162).

^[1] See Miri Rubin, Mother of God: A History of the Virgin Mary (New Haven; London: Yale University Press, 2009), 94; Pietro Sorci, ‘Maria nelle liturgie latine’, in Storia della mariologia, vol. 1, Dal modello biblico al modello letterario, ed. Enrico Dal Covolo and Aristide Serra (Rome: Città nuova; Marianum, 2009), 382–423: 412.