In the Chapter Library at Exeter, there is an unpublished fragment which comes from a Mary Missal; a Missal containing only the Ordo Missae and the Masses of our Lady (analogous to the Requiem Missals we have nowadays). They were produced for use in the Lady Chapels of great churches which were able to provide an entire establishment ... clergy, books, vestments, plate, cantors ... for use in such chapels. This fragment was corrected by the hand of Bishop John de Grandisson (pronounced Grahnsn; Bishop of Exeter in the fourteenth cenrury).
In this fragment, the Eastertide Mass of our Lady has a Sequence which neither the late Walter 'Patrimony' Frere nor I were able to find elsewhere. I print the remains below;  indicates a hole in the manuscript. The Revd Dr James 'Patrimony' 'Ordinariate' Bradley learnedly spotted that the first stanza came from the Mariale (III) by Bernard (of Cluny? or of Morlaix?); a long praise of our Lady in 15 Rhythmi; and the other stanzas are scattered around in V. I was accordingly able to fill up most holes in the text from Analecta Hymnica Medii Aevi; Volume 50, pp 423 sqq...
This hymn is associated with the name of S Casimir, because a text of it was buried with him. We know it best in the form of dear Fr Faber's translation Daily, daily, sing to Mary, which only uses a small selection from the hundreds of stanzas.
The question remains, whether the centonisation was done by Bishop 'Patrimony' Grandisson of Exeter himself, a great client of our Lady, for the Mary Missal which he had made for his Cathedral or for his collegiate foundation at Ottery; or whether someone else had already done the work.
At the bottom, for my own satisfaction, I give, from AH, a brief account of textual variants it records.
III.15 [O beat]a
per quam data
nova mun[do gaudia]
regna sunt caelestia.
V.19 [Quot dolore]s
tua sensit anima
cum in cru[cem
gens levavit pessima!
21 Corde tris[ti
28 Sed quam laeta
die statim tertia
[cum rex fortis
30 Nec nar[rari
tuum possit gaudium,
pacis dedit nuntium.]
mss=most of the manuscripts; ms=one or few of them.
III.15 ms quam beata
V.19 mss quot angores quot dolores; ms tot dolores; ms et dolores; ms quot languores quot dolores; ms quot dolores quot angores
mss cum; ms dum; ms levavit impia.
V.21 mss tuum cernis filium; ms cernens filium; ms tuum vides filium
V. 28 mss o quam; ms es completa; ms et repleta [haec cum assonantia]; mss die facta tertia; ms die virgo tertia; ms facta die tertia; ms dum rex; ms qua rex; ms dirae mortis; ms pertulit; ms iudicia.
V. 30 mss quis narrare quis pensare; ms quis pensare quis narrare; ms hinc narrare, quis pensare; ms unde maestis; ms quoniam maestis.