On the Origin of the Title, "Mediatrix of All Graces"?

The Catholic Church always has taught that Our Lord Alone redeemed mankind; and so, only through Him are salvation and Grace obtained. The term *mediator* means a "go-between." Thus, and in a sense, we are *mediators* when we pray to God on behalf of another person. Over the centuries, many Christians have believed the Blessed Virgin is the dispenser of all of the Graces which God deigns to grant; for example, St. Peter Damian: "In thy hands are all the Treasures of Divine Mercies." St. Bernardine of Sienna: "Thou art the dispenser of all Graces; our salvation rests in thy hands." And also, says St. Alphonsus Liguori, "was the doctrine of St. John Damascene, of St. Germanus, of St. Anselm, of St. Antonine, of Idiota, and of so many other learned authors..." Furthermore, although The Church has never solemnly defined this as dogma, the term "Mediatrix of All Graces" seems to have been used for centuries in reference to the Blessed Virgin Mary. As far as I can tell, the title dates back at least the fifth century, when Bishop Basil of Selucia made use of it. Prior to that, Church Father, St. Ephrem of Syria (4th century) writes: *"after the mediator, you (Mary) are the mediatrix of the whole world."* However, I'm not sure this means "mediatrix of all graces." QUESTION: When might the title, "Mediatrix of All Graces," first have been applied to the Blessed Mother; and, did any of the Church Fathers definitely make use of it (or something equivalent to it) to describe the Mother of God? Thank you.