Check out the facsimiles linked below Aristotles edition on the CPDL page (as you may have noticed, the Publication link below that under General info mistakenly links to the volume with the 8vv setting).

Okay, some might call that tripla. In Ari's score a (duple minim) = (three minims). In my score, a (duple semibreve) = (three semibreves). But the time signatures are C-slash (cut C) and 3/1 (in the source and in my score), the C-slash connoting two semibreves forming the duple structure. There is considerable ambiguity on the labeling of this relationship.

CH, yours is almost identical to the arrangement Roger Wagner used in his recording of the piece. I have no idea whose arrangement that is, however.

I have had my choirs sing early-music triple sections in tripla pretty much exclusively; I cannot recall an exception, anyway. For the piece in question, I also let my sopranos ignore the accidentals attached to "ora pro nobis" found in my CPDL edition. :)

For a Handl-to-Handl comparison, here's a screenshot of the Tenor part from the 1589 print of Obsecro Domine. The recording that persuaded me to put this in the Advent repertoire is sadly no longer available on YouTube, but I do remember its singers executing its conclusion in tripla (and adding a few melodic ornaments here and there to good effect).