Eureka

It must have been an illumination vouchsafed by the Paraclete, I say in all humility: on New Year's Eve at the parish vigil Mass, I finally understood why, at O.F. Mass, the first reading on the Solemnity of Mary Mother of God, Maria Mater Dei, is from Numbers 6:22-27, specifying the manner in which to impart the Aaronic benediction.

I had been trying to fathom the intended connexion between that pericope and the Gospel passage from St Luke 2:16-21, when it struck me: in Christ, God unveils and reveals His Face to us, as we gaze in spirit upon the Babe of Bethehem, just as the shepherds did in the flesh, having heard the angelic hymn of glory to God and peace to men of good will; and this was promised by the Aaronic blessing "May the Lord uncover his face to you and bring you peace" (Num. 6:26, J.B.).  For as Truth tells us, "He that hath seen Me hath seen the Father" (St John 14:9).

The Aaronic blessing is therefore a promise and prophecy of Him Who is to come.  Now, having looked up the readings of the solemnity in my old new missal, I turn to the Neo-Vulgate (which hasn't been lifted off the bottom middle bookshelf in quite a while), and transcribe this text:

22 Locutusque est Dominus ad Moysen dicens:

23 «Loquere Aaron et filiis ejus: Sic benedicetis filiis Israël et dicetis eis:

24 "Benedicat tibi Dominus et custodiat te!

25 Illuminet Dominus faciem suam super te et misereatur tui!

26 Convertat Dominus vultum suum ad te et det tibi pacem!"

27 Invocabuntque nomen meum super filios Israël, et ego benedicam eis».

Thus shall the sons of Israel be blessed, by Aaron and his sons their priests down the ages; thus shall God's Name ineffable be invoked upon the sons of Israel, for He promises that He will Himself bless them when such is said, as on earth, so in heaven.  Thus was God's commandment unto Moses.

But "the Law was given through Moses, grace and truth through Jesus Christ" (St John 1:17):  this verbal blessing is fulfilled in the Incarnate Word, Whose beauteous Face was revealed to us when born of Mary the Virgin at Bethlehem of Judæa in the days of Herod the King.  He it was Who in His own Person has enlightened us by the radiance of His Face shining upon us and having mercy upon us, as He did in His ministry, as He did on the Cross.  He it was Who turned His Countenance towards us and gave us His peace, peace such as the world cannot give.

Deus misereatur nostri et benedicat nobis; illuminet vultum suum super nos...

benedicat nos Deus, Deus noster, benedicat nos Deus... (Ps 66(67):2, 7b-8a, Nova Vulgata)

May God be gracious to us and bless us, and make his face to shine upon us... 

God, our God, shall bless us. God shall bless us... (Psalm 66(67):1, 6b-7a, English Standard Version)

Not for nothing did the ancient Fathers regard this passage of the Psalms, no less than the Aaronic benediction itself, as manifestly Trinitarian; not for nothing did Luther and the Lutherans adopt both one and the other as fitting formulæ of blessing.  Strange, how Catholic he could be, old Luther.

Neale, in his great Commentary on the Psalms, says of verses 6 and 7:

The mystery of the Holy Trinity is shadowed out in the triple recitation of the Name of God.  God the Father, Unbegotten, Underived, shall bless us.  Our own God, our Brother, God the Son, made like unto us, shall bless us.  God the Holy Ghost shall bless us. And the singular verb and pronoun which follow [in "and all the ends of the world shall fear him"] express the Unity.

All glory be to God!