On older writing instruments the absence of a serial number is no sign of a fake. Montblanc began numbering the fountain pens, ballpoints and propelling pencils of the Meisterstück series consecutively only from about 1991; the number appears on the gold band below the cap top with the star. Pieces made before that change carry nothing there but the country of origin.
That marking allows the age to be narrowed down roughly: “W. Germany” points to manufacture before reunification, that is, before 1989/90, whereas later pieces carry only “Germany”. A ballpoint marked “W. Germany” without a number and a propelling pencil marked “Germany” with a serial number can therefore both be genuine, but they then come from different decades.
What does decide the question of authenticity is the workmanship and the details: cleanly made trim with no bent sheet-metal parts, a correctly seated cap star, crisp engravings and the characteristic look of the precious resin. If doubts remain, a Montblanc boutique or a specialist workshop will examine the piece.