The prefix 'S' (e.g. S/0503) stands for 'Service' and marks writing instruments whose cap was exchanged at the factory; since for service no consecutive original numbers are reserved, these replacement caps receive their own S numbers. Prototypes carry the designation 'Sample' with three-digit number and are delivered, among others, to boutiques and country representations. With Limited Editions the total edition is always the sum of single sale and sets: example Marcel Proust: 17,000 fountain pens and 16,000 ballpoints as single pieces plus 4,000 sets yield consecutive numbers up to 21,000 or 20,000 respectively. The Hemingway formed an exception without total numbering and without set, since the ballpoint was only later produced at the request of the US market.
Related pages on fountainpen.de:
• Montblanc Writers Edition Hemingway (1992): https://www.fountainpen.de/writer-hemingway-en.htm