The stamp “W. Germany” or “W.-Germany” on the cap ring marks a pen out as a product of the period before 1990, made in the Federal Republic before reunification. After that the “W” for West was dropped, and the engraving has read simply “Germany” ever since. The reverse does not follow: “Germany” on its own is no reliable proof of a recent pen, since both spellings already occur side by side in the 1980s.
A second dating feature is the individual identification number, which Montblanc has laser-etched onto its pens only since about 1990. If it is missing altogether, that too argues for an older pen, and equally so whether the pen is a fountain pen, a ballpoint or a rollerball.
For a finer placing within that span, only the shape, the number and width of the cap rings, the lettering on the nib and the mechanism will take you any further. With production models an exact year cannot as a rule be established without a receipt; dating to the decade is both usual and honest.