This Alleluia is for the common Mass for feasts of a virgin martyr, and the text is from 2 Corinthians 11:2:

Emulor enim vos Dei emulatione despondi enim vos uni viro virginem castam exhibere Christo

For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin, to Christ.

A similar manuscript is in a university library in Slovakia:
https://cantus.sk/chant/28370

For many years I have had two large pages (about roughly 14x18 inches) which are difficult to put a date on. One, the better of the two, is probably XVI. century. The other, which is written with Roman letters, could be XVI to XVIII century. An irksome trait on both (does any one know the approximent date on which this degenerate practice began?) is that what would be podati are written as two left to right neumes joined instead of one over the other as an actual podatus would be written. Both have five line staves. The better of the two is Magi viventes, a Magnificat antiphon at vespers of the Epiphany. The text of the later one, Beatus Didacus, also a vespers antiphon, I haven't been able to assign a day for.

The other two are the really beautiful ones. They are about 10x12 inches and the pages are elaborately decorated and the lettering quite carefully and clearly written. They each have four line staves. I am of the opinion that they are probably XV century, perhaps a little earlier or a little later. The text of each are antiphons for the Exaltation of the Holy Cross.

It is always breathtaking to view something of great antiquity that looks like it was printed yesterday and has been well cared for over the centuries - such as that on Sam's example above.

Many thanks for this, Daniel!
Despite having his name right in front of me, I had not heard of St Didacus before and found that he led a very holy life as a friar healing the sick and those with challenging disorders.