On the feast of St Dominic last year, we posted notice (via the website Dominican Liturgy) of a very high quality scan of the Humbert of Romans Codex (Rome: Santa Sabina MS XIV L1), the manuscript which served as the prototype for the medieval (and early-modern) Dominican Liturgy. (Also available at https://archive.org/details/rome_santa_sabina_xiv_l1) Our friend Fr Innocent Smith OP, a scholar of Dominican chant and liturgy has just recently posted scans of an even older breviary from the same archive, which is kept at the Order’s mother house of Santa Sabina in Rome (Santa Sabina, XIV L2). Fr Smith informs me that this is the earliest Dominican breviary, dating from the middle of the 13th century, to include musical notation of the Office chants. It therefore slightly predates the reform of Humbert of Romans, which gave a definitive stamp to the Dominican Use; there are a few other pre-Humbert chant sources for the office, but this one is especially important since it also has the breviary texts in addition to the chants. It can be seen and downloaded at the following link: https://archive.org/details/rome_santa_sabina_xiv_l2_202401
Note the file is quite large, almost 600 JPGs at 3008x2000 pixels each, totaling 2GB, so it may take some time to download. Here are a few sample pages.The calendar pages of February and March. The entire calendar is in a different hand from the main body of the breviary, which indicates that it was added to the book later; the feast of St Thomas Aquinas has been added to March 7, sometime after his canonization in 1323.