Laetabimur in salutari tuo

The communion antiphon in the ordinary form for this weekend is Laetabimur in salutari tuo:



Text: We will rejoice in thy salvation: and in the name of the Lord our God we shall be exalted.

It is one of the more melismatic of the communion antiphons, and gorgeous beyond belief. The incipit alone is stunning, rising like incense. And then the exuberance grows through the second phrase that reaches to its highest point at "-tari" at which point we've already covered a full octave and a half step. As the melody proceeds, it continues with this exuberance, going way beyond the minimum necessary to make the structure work. The entire song suggests extension, elation, even spiritual hysteria of sorts (though contained, as chant is).

But I have a question about this that I can't seem to figure out. Unless I'm missing something, this is not listed in the Liber Usualis as a Communion antiphon. Was it not in the old calendar? What's going on with this piece, and where did it come from? Does anyone know? (Coda: Questions answered in comments)