Packaging and accompanying papers are by now imitated just as the writing instruments themselves are; counterfeit boxes and certificates are in some cases barely distinguishable from the original. A listing with complete accessories therefore says nothing about authenticity.
The same applies to the serial number on the clip. A few years ago it could still be read off it fairly reliably, in the case of plain Meisterstück pens, whether an original was at hand; by now counterfeits also bear an engraved number. Conversely, some genuine writing instruments have no serial number at all, among them older models as well as the Rouge et Noir, the Noir et Noir and the writing instruments of the Scenium series. A missing number is thus no proof to the contrary, and a number that is present is no proof of authenticity.
A Montblanc refill in a ballpoint pen likewise proves nothing in itself, for refills in particular are frequently counterfeited.