|St John the Evangelist. Initial "U". |
[Source: Christie's "online only" auction.]
The illuminator is very curious from a connoisseurship point of view, because of the way his style changed during his career: there is a very Bolognese late 14th-century style, clearly following that of Niccolò da Bologna; there is a modified style when he worked in Paris, c.1400, on such manuscripts as the Brussels Hours of the Duke de Berry from which he takes his name ; and there is a third style, represented by several manuscripts including the present Gradual cuttings and a manuscript written in Bologna in 1408. It is therefore assumed that he trained and worked in Bologna; moved to Paris around 1400; and returned from Paris to his home-town between about 1405 and 1408.
The seven previously-known Gradual cuttings were listed by Pia Palladino in her Treasures of a Lost Art exhibition catalogue, no. 34 :
A Prophet(?). Initial "A". Venice, Cini Collection, Inv. 2048:
A Prophet(?). Initial "I". Amsterdam, Rijksmuseum, RP-T-1937:1:
St Dominic. Initial "E". Formerly Edward Holmes; later Maggs, Bulletin 1: European Miniatures and Illumination (Feb. 1962), no. 6:
St Stephen. Initial "S". NYC, The Met, 31.134.1:
Christ Washing the Apostles' Feet. Initial "D". Formerly in the Holford Collection:
Christ Blessing. Initial "E". Formerly in the Lehman Collection at The Met, subsequently with Jörn Günther:
"[Ante me non est formatus deus et post]If this is correct, then the initial "E" itself, depicting Christ surrounded by cloud-like white on a blue sky-like background, probably introduced the chant "Ecce apparebit dominus super nubem candidam ...", the first response at Matins for the Third Sunday in Advent.
[me non erit quia michi cur]uabitur
[omne genu et confite]bitur omnis
St Nicholas of Bari. Initial "C". Formerly Christie's, 26 June 1991, lot 12; now at the Museum of the Rhode Island School of Design:
"Confessor dei Nicolaus nobilis progenie sed nobilior moribus ab ipso puerili aevo secutus dominum meruit divina revelatione ad summum provehi sacerdotium ...",followed by "Erat enim valde compatiens ...".
To the seven previously-known cuttings, and the eighth now at Christie's, I can add one more:
The Nativity. Initial "H" (presumably "Hodie natus est ...") . Fritz Roth, Lausanne, Catalogue 9: Catalogue de très beaux livres , no. 12:
The cuttings with identifiable feasts therefore include a clutch from November and December: 
- 6 December. St Nicholas
- 3rd Sunday in Advent
- 25 December. Nativity
- 26 December. St Stephen
but others come from Holy Week (the Christ Washing the Apostles' Feet) and August (St Dominic).
EDIT 24 Jan 2018
I have found that the St Nicholas of Bari cutting was in Maggs Bros.'s European Bulletin No. 17 (1992), no. 8, with a colour image on the inside back cover, already marked as "Sold", and the Nativity cutting was in Maggs Bros. Catalogue no. 642, Illuminated miniatures and original drawings (1937), no. 19:
 I am not persuaded by the identification of the artist as Giovanni di fra' Silvestro; see Massimo Medica, "Un nome per il 'Maestro delle Iniziali di Bruxelles': Giovanni di fra' Silvestro", Arte a Bologna: Bollettino dei Musei civici d'Arte Antica, no 7-8 (2011), pp. 11-22.
 This is now available to download free from here.
 I also suspect that the Cini initial "A" may be the First Sunday in Advent, "Ad te levavi", as it is larger than at least some of the others. The recent Cini exhibition catalogue states that the back of the cutting has text in brown ink in gothic script, and music in square notation on four-line red staves, but unhelpfully states that "il ritaglio non permette l'individuazione del testo a cui l'iniziale era a capo", without recording what the visible text is!