A systematic attempt to catalogue the Additional Manuscripts was first undertaken by M. R. James in 1926–1930 as part of a larger plan laid down by the Librarian, A. F. Scholfield, to produce a new catalogue of the whole of the University Library's manuscript collections. James had agreed to catalogue all saving the legal manuscripts, or anything later than 1500. In the case of cartularies and the like, he intended simply to revise the printed descriptions. James completed the task as far as Add. 6657. Most of the work was finished by 1930, but there were many loose ends to be tied regarding the manuscripts cataloguing project as a whole, and the separate question of what should be done about the legal manuscripts was not to be resolved until Sir John Baker’s catalogue of 1996. This was published by Boydell and Brewer as J. H. Baker, A Catalogue of English Legal Manuscripts in Cambridge University Library, with codicological descriptions of the early manuscripts by J. S. Ringrose (Woodbridge, 1996).

James was paid for his work in 1933; in 1934 the Library moved to its new building on the present site, and James received an honorary degree. In 1936 he died. An attempt to prepare James’ descriptions for publication during the Second World War failed when the University Typewriting Office had to give up trying to decipher James’ pencilled drafts.

Nonetheless, during the War, further plans were mooted to publish a complete catalogue of all the University Library's manuscripts, to replace that published in the mid-nineteenth century. In 1948, Mr H. L. Pink, who had recently been appointed Assistant Under-Librarian, began to work on the revision of James’ descriptions. He found the work arduous, and progress was slow. It turned out that James’ descriptions were not suitable for mere revision, and he had to begin the work de novo. Sir Roger Mynors, Kennedy Professor of Latin 1944–1953 and a Library syndic, was encouraging and laid down guidelines. Mynors himself undertook descriptions of about one third of the Additionals, picking out the classical and renaissance texts, together with the earlier medieval manuscripts, which reflected his own interests and expertise. He and Pink also did much work on the two-letter classes in the same way.