Thank you for the answer. Although I'd say the exact contrary of what Norberg says regarding the elisions in Ambrosian hymns (i.e. hymns that are ascribed traditionally to Ambrose even if most of them aren't probably of Ambrose), because lately I've become an Ambrose-aholic (also thanks to a conference I've attended regarding Ambrosian Hymns).
I could quote a couple of them, in case some Ambrose-aholic comes here and wants to have fun:
Ut martyrum vestem attigit
et ora tersit nubila
lumen refulsit illico
figitque pulsa caecitas
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Apostolorum Passio
Praecinctus, ut dictum est, senex
et elevatus ab altero
quo nollet ivit, sed volens
mortem subegit asperam
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Apostolorum supparem
Apostolorum supparem
Laurentium archidiaconum
pari corona martyrum
Romana sacravit fides
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Splendor paternae gloriae
Splendor paternae gloriae
de lucem lucem proferens
lux lucis et fons luminis
dies dierum illuminans (a verse you found in fery few codexes, usually you find diem dies illuminans, but the critical edition of Fontaine picks the "lectio difficilior")
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Iam surgit hora tertia
Hinc iam beata tempora
Christi coepere gratia
fide replevit veritas
totum per orbem ecclesias
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Intende qui regis Israel
Intende, qui regis Istrael
super Cherubim qui sedes
appare Ephraem coram excita
potentiam tuam et veni
(in the Roman Lithurgy, they thought it wasn't possible to force here the beginning of Psalm 71, so they deleted this strophe. The Milanese rite is still sung pure to this day)
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Non ex virili semine
sed mystico spiramine
Verbum Dei factum est caro
fructusque ventris floruit
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Amore Christi nobilis, the one I asked about at the beginning, because I were reading them and realized something was wrong. In fact this hymn was deleted from the Ambrosian Breviary in 1600 and republished only later for the Liturgia Horarum renovata
In principio erat Verbum
et Verbum et cetera et cetera
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Illuminans Altissimus
Illuminans Altissimus
micantium astroum globos
pax vita lumen veritas
Iesu, fave precantibus
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Vel hydris aquae
vini saporem infunderis
hausit minister conscius
quod ipse non impleverat
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Victor Nabor felix pii
Scutum viro sua est fides
et mors triumphus, quem invidens
nobis tyrannus ad oppidum
Laudense misit martyres
__________________
Grates Tibi, Iesu, novas
His quis requirat testium
voces ubi factum est fides?
Sanatus impos mentium
opus fatetur martyrum
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So if I apply elision these verses are perfect iambic dimeters. So I personally reached the conclusion Ambrose wouldn't have them sung with extra syllables, otherway he wouldn't sistematically use this metric device. In the 1800 Emilio Garbagnati and Luigi Primo Colombo published a new edition where the extra syllables haven't the music notes (the medieval codes have the extra notes, so they were certainly sung as you suggest back then). Source: a conference in the Ambrosian library, I can't remember the name of the professor I'm afraid! But I don't want to steal his credits.