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 mid-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.