Liber misticus (type of book containing chants, readings and prayers for the cathedral ordo, i.e. vespers, matutinum and mass, and on penitential days only, also tertia, sexta and nona) with material for Post-Pentecost time offices from 16 June (Saints Adrian and Natalia) to 12 November (Saint Emilianus). This manuscript has the most complete rendition of these offices in the Hispanic rite. It consists of 19 quires, which are preceded and followed by extra folios. The quires have 8 folios each, except the last quire which is incomplete (with only its initial four folios). These quires are numbered up to quire XVII on f. 140r. Between quire X and XI (ff. 89r–93v) there is an extra 4-folio quire, written and decorated by other hands, containing the prayers for the mass of the Assumption of Mary. Before the first quire which starts on f. 8r there are parts of the offices of Saint Quiricus (f. 1r-2v), Saints Vincent and Leto (ff. 3r-v), and Saint Jerome (ff. 5r-7v, almost complete), and a notated Credo (ff. 4r-v). After the last quire (which lacks its final 4 folios, corresponding to the end of Canonical Litanies), there are two folios containing the end of the office of Saint Christina and almost all of the office of Saint Bartholomew. The fact that the initial and final folios include incomplete and displaced renditions of the feasts they transmit – on the the initial folios we find parts of Saint Quiricus, observed on 13 June, followed by a folio from Saints Vincent and Leto, observed on 1 September, and folios from Saint Jerome, observed on 30 September, according to the Hispanic rite calendars – suggests that these folios were probably serving as flyleaves or protection for the main body of the manuscript, rather than intended to provide material that was missing in the main manuscript. This is compatible with the hypothesis raised by some authors, including Janini (1978) and Díaz y Díaz (1980 and 1983), among others, that the initial and final folios (ff. 1r–7v, and 160r–161v) originally belonged to other books; these authors mention palaeographical features that distinguish these folios from the rest of the manuscript. As noted by several authors, this manuscript also contains another notated fragment (ff. 84r-v), though it is not clear if the notation was added before or after the rest of the manuscript was completed.
This manuscript has very interesting monograms preceding each office that have a style that is not present in any of the manuscripts currently preserved at Santo Domingo de Silos: some of them have human shapes, representing the celebrated saints, other have animal or vegetable shapes.
Though the musical notation is of the vertical variety several neumes are tilted towards the right as is typical of the horizontal notation. This is also one of the manuscripts with what Brou (1955) called "intermittent notation" (i.e. when a syllable bears one note only sometimes such note is not copied).

Date: most authors place its production in the tenth century; others date it as tenth-eleventh century (Férotin, 1912; Rojo and Prado, 1929), end-tenth century to beginning-eleventh century (Boynton); eleventh century (Walker, 1998; Millares Carlo, 1999). Díaz y Díaz (1983) dates: ff. 1r-7v as end-tenth century, the fragment in ff. 86 bis r-v as tenth century, ff. 91r-94v as eleventh century, and does not give a date for the rest of the manuscript but mentions that it is considered as the continuation of GB-Lbl Add Ms 30844, which he dates as early eleventh century. Janini (1978, with the help of Mundó) dates the initial and last folios as tenth century, and ff. 8r-159v as eleventh century.

Origin: the British Library (when it was British Museum) acquired it from Quaritch, which had bought it from the Abbey of Santo Domingo de Silos in the late nineteenth century. As noted before, this manuscript does not resemble any tenth or eleventh century manuscript preserved there. Boynton and Whitehill proposed San Millán as its place of origin. As Boynton ('A lost Mozarabic manuscript', p. 191) explains,'It was apparently produced in San Millán' (my italics) mainly based on the fact that 'the mass for San Millán refers to him several times as "noster" and "patronus"'. Although this attribution is possible, the musical notation, script, codicology and miniatures in this manuscript, suggest that this manuscript was produced elsewhere, and this becomes even more apparent if the whole corpus of Hispanic rite manuscripts are examined in person. San Millán was a powerful motherhouse and had a large network of convents and monasteries linked with it, all of which would call San Millán their patron. Furthermore, especially after the battle of Simancas in 939, any monastery or church in La Rioja, and the Castilla and León areas, was likely to call San Millán their patron, and there are many big scriptoria documented from this area (Tábara, Nájera, Albelda, Cardeña, Oña, etc...). Díaz y Díaz (1983) suggests that what he calls the main part of the codex is from Santo Domingo de Silos, but not from its monastery but from a place of exchange; he later (1995) adds that it has symptoms of the South of the Península.

Liturgical tradition: A.

Randel responsorial tone tradition: [La] Rioja, though "it shows a greater number of León symptoms" if compared with GB-Lbl Add MS 30851 (Randel, 1973, p. 70).