From the OED archives: ‘addendum’

This slip was one of those written by Sir James Murray himself. It details the etymology, pronunciation, and the first part of the definition of sense 1, of addendum. The tramlines to the left of the headword indicate that at the time of writing, Murray felt that addendum should be regarded as a loanword and that it had not yet been fully assimilated into English. The entry has since been revised (and the word is now considered part of standard English), but Murray’s work can be seen by clicking on ‘Previous version’when you visit the entry.

What does the number 456 at the top of the slip mean? Every dictionary slip carried such a number, so that if an editor dropped a bundle, the slips could be reorganized easily.