In the Missal of Ioannes XXIII; this began in the main post for Saint Ambrose but eventually it seemed to deserve its own.
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The Missal, sub Ioannis Conciliarii Progenitoris regimine, also provides an alternate Mass for Saint Ambrose in its section Pro aliquibus locis, Optavi et datus est. It doesn't say which the 'certain place' is or 'places' are, or give any information about the sources of the texts. That Introit antiphon is provided as an ad libitum one in the Common of Doctors in the Antiphonale Monasticum of 2007, but isn't in the older Antiphonale that I've got here (from 1935) and isn't in the Graduale Triplex from 1979, either; the Mass formulary must (obviously) precede the post-conciliar liturgical vandals, however, so I'll add it here. Perhaps it was or is in use amongst the Augustinians; who knows, although it seems familiar, for some reason, Optavi et datus est. Perhaps Dr DiPippo will enlighten the darkness.
The thing to do is search in the Missals at Usuarium, ahem, all dozen or so, to find this lesson and perhaps these parts of the proper. Eh.