"I think the XXI s. is referring to the text, that has been invented for the modern Liturgy of the hours.  I have other books on Dominican chant, and I think that the dates of the melodies come from the dates of the ancient Dominican manuscripts."

"13th c.
https://web.archive.org/web/20160315065444/http://hymnarium.org/mel...
(melody 56)"

"I would suspect that "Nabuchodonosor Rex" would usually be sung to the Prophecy tone, as suggested in various Liturgical Books.  I have read somewhere, Liturgical Year, Gueranger? there was a tradition that this being the last of the 12 Prophecies…"

"All the sequences...  A look at the Analecta Hymnica will show how many there are.
https://archive.org/search.php?query=analecta%20Hymnica%20AND%20med...
Vol. 8-10, 34, 37, 39-40, and 44 are for Sequences text only.

Books with the melody are more…"