Advent responsory: Rain on a fleece as a symbol of the Incarnation

Today's responsory is the third for Tuesday in week three of Advent (also the seventh of Sunday), and contains some rich imagery of the Incarnation.  The text is largely taken from Psalm 71:

R. Descéndet / Dóminus sicut plúvia in vellus: * Oriétur in diébus eius iustítia, et abundántia pacis. V. Et adorábunt eum omnes reges, omnes gentes sérvient ei. R. Oriétur in diébus eius iustítia, et abundántia pacis. V. Glória Patri, et Fílio, et Spirítui Sancto. R. Oriétur in diébus eius iustítia, et abundántia pacis.

R. The Lord shall come down like rain upon a fleece. * In His days shall righteousness flourish, and abundance of peace. V. All the kings of the earth shall fall down before Him, all nations shall serve Him. R. In His days shall righteousness flourish, and abundance of peace. V. Glory be to the Father, and to the Son, and to the Holy Ghost. R. In His days shall righteousness flourish, and abundance of peace.

St Augustine's commentary on this psalm links it to the story of Gideon and the fleece in Judges 6:

He has called to our minds and admonished us, that what was done by Gedeon the Judge, in Christ has its end. For he asked a sign of the Lord, that a fleece laid on the floor should alone be rained upon, and the floor should be dry; and again, the fleece alone should be dry, and the floor should be rained upon; and so it came to pass.  

Which thing signified, that, being as it were on a floor in the midst of the whole round world, the dry fleece was the former people Israel. 

The same Christ therefore Himself came down like rain upon a fleece, when yet the floor was dry: whence also He said, I am not sent but to the sheep which were lost of the house of Israel.  There He chose out a Mother by whom to receive the form of a servant, wherein He was to appear to men...

The earliest responsories?

Most of the responsories we've looked at so far this Advent have been either non-psalm based, so this is an interesting example of a distinct set of psalm responsories clearly chosen for their appropriateness to the season or feast.

And indeed, one of the oldest possible references to the responsory repertoire relates to exactly this type of responsory.  Gennadius of Marseilles wrote about some work commissioned by his predecessor Venerius (428-52), saying:

Musaeus, presbyter of the church at Marseilles, a man learned in Divine Scriptures and most accurate in their interpretation, as well as master of an excellent scholastic style, on the request of Saint Venerius the bishop, selected from Holy Scriptures passages suited to the various feast days of the year, also passages from the Psalms for responses suited to the season, and the passages for reading. 

The readers in the church found this work of the greatest value, in that it saved them trouble and anxiety in the selection of passages, and was useful for the instruction of the people as well as for the dignity of the service.

He also addressed to Saint Eustathius the bishop, successor to the above mentioned man of God, an excellent and sizable volume, a Sacramentary, divided into various sections, according to the various offices and seasons, readings and psalms, both for reading and chanting, but also filled throughout with petitions to the Lord, and thanksgiving for his benefits.

By this work we know him to have been a man of strong intelligence and chaste eloquence. He is said to have also delivered homilies, which are, as I know, valued by pious men, but which I have not read. He died in the reign of Leo and Majorianus.

It is unclear in this, of course, whether he is talking about the Office was well as the Mass, particularly as the Gallic equivalent of the Roman Gradual was called a responsory.  Still, the comment seems broad enough to encompass both uses of psalms.