The Sir Georg Solti Donation Pen carries no engraved serial number, so examples without a number are nothing to take exception to. According to collectors, the only Donation Pen to bear such a marking is the Johann Sebastian Bach. If the corresponding field on the warranty card is left blank when buying from a specialist shop, that follows logically and is not a fault, since the dealer cannot enter something the pen does not have.
Nor are the Donation Pens numbered limited editions; they are a series produced over a longer period in aid of cultural projects, which explains the absence of an individual number. Beyond that, the serial number is not a consistent feature at Montblanc at all: several series were delivered without any such engraving, among them the Rouge et Noir, the forerunner of the Bohème line. No recognisable system behind it has come down to us, so neither the authenticity nor the age of an instrument can be inferred from the presence or absence of a number alone.