I have just re-discovered this leaf in Friedrich Georg Zeileis, "Più ridon le carte": Buchmalerei aus Mittelalter und Renaissance: Katalog einer Privatsammlung von illuminierten Einzelblättern, 3. rev. Nachdruck der Aufl. von 2004 (Rauris, 2009), no.145:
which also reproduces the recto of the leaf:
"Familiam huius /
sacri cenobii quesumus domine interce/
dente beata dei genitrice /
Maria et beato Benedicto con/
fessore tuo cum omnibus sanctis tuis ... "
This is a very rare collect in Psalters and Books of Hours; elsewhere it sometimes has the rubric "Pro congregatione", and sometimes inserts the name of the monastery's name-saint(s) with, or instead of, Benedict's, as in the Westminster Missal:
part of BL, Cotton MS Tiberius A.iii, probably from Christ Church, Canterbury:
and the Fulda Sacramentary:
It is a pity that the scribe of our Psalter-Hours did not insert the name of the nunnery's name-saint, but the inclusion of this rare collect gives hope that there may be other helpful textual clues waiting to be found, that will help identify the (very uncertain) origin of the manuscript.