From inchoation to perfection

"O God, who in your kindness begin all good things and bring them to fulfillment, grant to us, who find joy in the Solemnity of the holy Mother of God, that, just as we glory in the beginnings of your grace, so one day we may rejoice in its completion.  Through."

"Deus, qui bona cuncta inchoas benignus et perficis, da nobis, de sollemnitate sanctae Dei Genetricis laetantibus, sicut de initiis tuae gratiae gloriamur, ita de perfectione gaudere.  Per."

     Apart from the 20th-century insertion of "de sollemnitate sanctae Dei Genetricis laetantibus" (1 January), this is taken word for word from the "Leonine" or Veronese sacramentary (and, according to Corpus orationum, no subsequent!), where it (no. 1006 in the modern critical edition) functions as a post-communion "in natale episcoporum" for the month of September.  The pre-1956 scholarship cited by Mohlberg dates it to between 537 and post-560, depending, attributing it to Leonine (?) anti-semi-Pelagianism, to Vigilius, to Pelagius I, etc.