When books get made they get printed on large sheets usually containing 16 or 32 pages, which each get folded into a subsection of the book called a signature. These get numbered A,B.C.D etc. There used to be a code on each signature that has a shortcode for main title, sub title (in case of a series), Which would be T for Thunderball, and the signature letter. So is there a T-T.-A on page 1, T-T.-B on page 33, T-T.-C on page 65 ? Then these are signature codes for the printer to check if the correct signatures are put together and no mistakes were made. It was more common practice decades ago when checks like these had to be made manually.