From Wikipedia: A signature mark is a letter, number or combination of either or both, which is printed at the bottom of the first page, or leaf, of a signature or section. This practise is to ensure that the bookbinder can order the pages and sections in the correct order.
If you like to research old books you have probably seen thousands of signature marks without realizing what they were. Just look for the little “R” or “B2″ or whatever down at the bottom of certain pages. Most older books were printed as big sheets that were later folded and bound. The printer was extremely careful to arrange his work such that as the sheets were folded into (generally) 16 pages, the marks were exactly in place. Those folded sheets, now called signatures, were then easily matched together for the binding process. Some old printers—such as myself—were sometimes caught incorrectly saying “registration marks” or “printer marks”. Whatever the name, signature marks have no place in digital documents, such as pdfs, so I routinely remove them when processing a scanned document. It is just another little service needed to make precious old books more readable to a new generation.








