159v and 213r
Folios 159v (Figure 1) and 213r (Figure 3) both contain pasteovers at the conclusion of tracts: "Attende celum" for the vigil of Pentecost on 159v, and "Qui seminant" for the Feast of Saints Fabian and Sebastian when it falls in Septuagesima on 213r. It is unclear, but seems likely, that they are notated in the same hand, since both pasteovers transmit virtually identical music.
How does one tell to which chants the addenda belong? Folio 159v provides a clue: the last few neumes for the "Attende celum" tract are identical to the first few of the marginal addition. (See Figure 2, opposite.)
Tracts are highly formulaic chants. Consultation with Emma Hornby's book, Gregorian and Old Roman Eighth-Mode Tracts (Aldershot: Ashgate, 2002), revealed that the alterations made to the melodies on both folios conform to standard Gregorian versions of the chants, extant since the early middle ages. This suggests that the original scribe of Western MS 97 may have been working from a faulty exemplar, which these emendations later corrected.
In sum, neither addition leads us any closer to our questions about the date or place of origin of the manuscript.