At the bottom of the page the signature mark represents the number of the leaf.
A signature mark, in traditional bookbinding, is a letter, number or combination of either or both, which is printed at the bottom of the first page, or leaf, of a section. (The section is itself often known as a "signature", although technically this usage is incorrect.) The aim is to ensure that the binder can order the pages and sections in the correct order. Often the letters of the Latin alphabet have been used. The practice has been overtaken by advances in printing technology, and signature marks are rarely found in modern books.