The leading "S" stands for "Service" and indicates that the writing instrument has already been at Montblanc for repair once. Since for every limited edition a complete spare-parts inventory cannot be kept, exchanged parts receive this special S marking. Specifically, only caps are given S numbers, since the limitation number is engraved there; on non-limited writing instruments no such limit number exists, only the production coding.
Related pages on fountainpen.de:
• Montblanc Writers Edition Agatha Christie (1993): fountainpen.de/writer-christie-en.htm