The host volume is a Psalterium feriatum, with antiphons and hymns, written in 1545 (colophon f. 102v) by the organist and calligrapher Fridolin Sicher (1490-1546). Other scribes added to the Psalter after 1545. At some point, ff. 145-149, (by Sicher) were removed.
One of these removed leaves, f. 149, is used as the rear pastedown; another fragment was glued half under the front pastedown; the current ff. I-II were attached to the fragment, and the exposed part was trimmed to a strip (V3).(Sandy Maillard, Fragmentarium, 2018, using material from Franziska Schnoor, M.A., Stiftsbibliothek St. Gallen and Beat Matthias von Scarpatetti.)
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St. Gallen, Stiftsbibliothek St. Gallen, Cod. Sang. 544, V3v rotated – Psalterium feriatum with Antiphons and Hymns — https://fragmentarium.ms/view/page/F-11ks/1569/19708