“Chanted Angelus” • Where does it come from?

ACK in 2010, I released this lovely version of the ANGELUS in Latin—which was a favorite of my high school students—on the CMAA forum. Since that time, I’ve been astonished by the number of times it has been downloaded and recorded by people across the globe:

    * *  PDF Booklet • THE ANGELUS (Dom Charpentier, OSB)

But did Dom Charpentier, OSB, truly compose it?

The melodic pattern is quite common in the Gregorian repertoire and would have been known even in the “decadent ages” of Gregorian chant (circa 1700). However, I recently came across this ancient manuscript:

This implies that Dom Charpentier created his rendition based on an ancient model.