Until 1913 there was a shelfmarking system of numbers and letters to indicate bookcases and shelves. These were written in the top right hand corner of the flyleaf, next to the accession number. This numbering was used while the library was at Snow Hill and was retained unchanged when the Library was moved to Wigmore Street in 1911.
T.W. Huck, the first professional Librarian (1913-1916), started an alphabetical classification but didn't get beyond class A in applying it. His numbers appear in the centre of the front pastedown.
C.C. Barnard, his successor (1919-1921), devised a new classification retaining Huck's alphabetical outline. This was developed by subsequent librarians and used from the whole Library until 1946. These shelfmarks are about a third of the way down the front pastedown. (The Barnard Classification for Medical and Veterinary Libraries still used by Wellcome Collection, and a handful of other specialist libraries, was first published by C.C. Barnard in 1936, when he was Librarian at the London School of Hygiene and Tropical Medicine.)
After the abandonment of the Huck/Barnard in-house classification the books were arranged by date and size and then alphabetically by author (or other heading) with no shelfmarks allocated.
Incunables were given their own shelfmarking system of numbers and letters. Originally this related to actual cases with the smallest books at the top (shelf 'a') and increasing in size to 'f' at the bottom, and books on each shelf in numerical order.
Some books have notes of subject headings at the end, in the hand of H.R. Priest, Assistant Librarian, 1906-1911.