No. Serial numbers, country markings such as Germany and other engravings are no proof of authenticity, since they can be imitated just as readily as the pen itself; fakes routinely carry complete numbers and imprints. Conversely, older originals have no serial numbers at all, as Montblanc has only been allocating them since the early 1990s. The only telling evidence is the execution as a whole – shape, fittings, rings, clip, surface finish and writing unit – compared against a reliable model survey. A combination of colours or materials that never existed in the edition concerned rules out an original, even if the mechanism and the overall look seem right at first glance; that applies, for example, to pieces modelled on a Muses Edition but turning up in a silver-and-gold combination that was never made.
The question can only be settled bindingly by the manufacturer. Montblanc accepts pens for examination, usually for a handling fee, and then issues a legally binding statement. This is particularly advisable where a purchase is to be unwound. Nor is provenance as a gift from a business contact any proof of authenticity, since that is precisely the route by which copies often enter circulation unnoticed.