I have come across a lot of commentary on the breviary reform of 1911 under Pope St. Pius X. Most of this commentary talks about the redistribution of the psalms. I haven't seen much comment, though, on the introduction of new antiphon texts, and the effect this would have had on the preparation of the 1912 Antiphonale Romanum which was incorporated into the Liber Usualis.

For an example, the psalms of Sunday Vespers (unlike Lauds and other hours) survived the reform intact, but there were significant changes to the antiphons, marking a sudden departure from a tradition that may have dated back to the Roman liturgy of the fifth century or earlier:

Pre 1911 Sunday Vespers

Ant. Dixit Dominus Domino meo: Sede a dextris meis + Ps. 109
Ant. Fidelia anima mandata eius: confirmata in saeculum saeculi + Ps. 110
Ant. In mandatis eius cupit nimis + Ps. 111
Ant. Sit nomen Domini benedictum in saecula + Ps. 112
Ant. Nos qui vivimus, benedicimus Domino + Ps. 113

Post - 1911 Sunday Vespers (differences in bold)

Ant. Dixit Dominus Domino meo: Sede a dextris meis + Ps. 109
Ant. Magna opera Domini: exquisita in omnes voluntates eius + Ps. 110
Ant. Qui timet Dominum in mandatis eius cupit nimis + Ps. 111
Ant. Sit nomen Domini benedictum in saecula + Ps. 112
Ant. Deus autem noster in caelo: omnia quicumque voluit, fecit + Ps. 113

This is just one example. Through googling, I found a post on a traditionalist forum from someone who had been asking the same questions I've been asking and who took the trouble to compile a table of the differences. This person demonstrates that 79 antiphons simply disappeared as a result of the 1911 reform, and 158 (!) new antiphons were introduced.

As this individual puts it: "Where the old music went or where the new music came from...is even more mysterious." We know that the timing of the final volume of the Vaticana, the Antiphonale, coincided with the new Breviary. Did the Breviary reformers of 1911 take their antiphon texts from newly discovered musical manuscripts? I rather doubt it, and searching these texts on the Cantus and Latrobe databases and for the most part coming up with zilch reinforces my suspicions.

It would appear - and if I'm wrong, someone please set me straight - that 1911 marked the beginning of a familiar pattern for liturgical reform, with those in charge introducing novel antiphon texts, with complete disregard for the chant repertory, often in a seemingly arbitrary, change-for-change's-sake manner.

Since the 1912 Antiphonale Romanum (and thus also the Liber Usualis) follows the breviary as far as these texts go, I have to assume that these are new compositions, less than a century old, unless the editors managed to find some obscure matching antiphons somewhere. Does anyone know for sure?

It's worth noting that the 2009 Antiphonale Romanum restores those missing vespers antiphons, and does not include any of the antiphons that first appeared in 1911.