According to collectors, an identification number beginning with “S/” indicates that the pen has been through Montblanc's service department and that something on the cap or clip was repaired or replaced there. On Limited and Special Editions the serial number sits on the cap or the cap ring; if that part is replaced, the original number cannot be carried over, and the service department marks the replacement part accordingly. The same applies to numbers prefixed with an “R”, and to replacement caps with no number at all.
A full overhaul in which the cap is kept, by contrast, does not result in such an addition — an overhauled pen with its original cap keeps its number unchanged. An “S” is therefore not a question of authenticity, but it does reduce the value of a collector's piece, because it is no longer entirely original. Anyone buying a numbered collector's piece should look at the number before buying and take such an added letter into account in the price.