it is likely written by the director of the schola. I have one that I lifted from a video under Massimo Palombella that was likely written by him. It's similar in style but notably different and had parts for brass too. You can generally get sheet music for things like this by contacting the appropriate office at the vatican. I have done this before, but don't remember what the contact info is. I'll take a look and see if i can find it.
Marier's "Hymns, Psalms, and Spiritual Canticles" has the simple chant melody, followed by the Dresden Amen (shades of Parsifal), with optional descant. Used this for a full decade and always seed just right. Lead right into the Lord's Prayer. Pastor was excellent at getting the pitch right.
Marier actually wrote three versions, the first two of which were both used with some frequency (the first includes the riff on the Dresden Amen), the last of which not much if at all in my times at St Paul's from 1983 (when the second and last edition of that hymnal was published and put into the pews, though I remember the red first edition for a while)-2012.
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