After 1990, Montblanc placed serial numbers on the ring at the top of the pen. Special characteristics like engraving, the rounded star emblem and barrel coloring mark authentic pens made prior to 1990.
The serial number is one way of authenticating Montblanc pens. Most pens come with a service guide certificate. Other means of authentication include engravings under the clip, on the top ring or nib. These include "Made in Germany" or "4810."
Another characteristic is the white, six point, rounded star on the pen's cap. Additionally, the pen's barrel will have a reddish tint when held up to the light. The best way to authenticate a pen and verify a serial number is to take it to one of the company's 449 boutiques.