Wow! What a mustached bookbinder! I just posted some images of decorated initials from Het drukkers jaarboek 1906 recently, but there is much more interesting in this series of books. And amazing book industry ads is one of these things.
Most of you probably used Excel or a similar spreadsheet software. If columns are set to be labeled by letters after Z goes AA. And then AB,... AZ, BA,... etc. But when it comes to old books, there were other ways to use letters for sorting.