Following the Mendizábal disentail, this codex came under the control of the registrar of León, Ramón Alvarez de la Braña, then went to the School of Diplomatics in Madrid and finally became part of the stock of the National Historical Archive in Madrid, where it is kept as item 1097B.
Recent critics maintain that this codex is made up of a large basic manuscript to which were added, in the Middle Ages, two folios from a Beatus from the San Salvador Monastery in Tábara. The origins of the first manuscript, which is made up of 166 folios measuring 360 x 255 mm., with Visigothic script in two columns, are unknown; all that is certain is that it is from León and can be dated to the 10th century, that is, the finest period for calligraphy and miniatures. As with other Visigothic codices, there are glosses in the margins in Arabic, indicating that the main language of some of the members of the monastic community was the Arabic of Moslem Spain. The manuscript has suffered what can only be described as savage mutilation, since only eight of the 100 or so miniatures usual in a Beatus are still present. Those which survived confirm the evidence of the script, that is that this work belongs to the great 10th century León School.
To this original text were added (probably in the Middle Ages) two folios from another Beatus (folios 167 and 168), which had to be cut as they were larger than the earlier ones. These folios have lent the whole codex its name, since it is on them them that appears the famous miniature of the Tábara tower, which depicts Senior and Emeterius and an assistant in the scriptorium of San Salvador Monastery. In the colophon at the end of the work below a monumental, decorated omega, it is written that the first copyist was the master Magius, but that he was overtaken by death and the monastic community then called on his pupil Emeterius to finish it, which he did on the 27th of July in the year 970, after three months of hard work.