Montblanc burned the code onto the clip only on the first series of the instruments; thereafter it is located on the upper cap band. It always consists of two letters followed by seven digits and is executed as a laser engraving, so that it appears burned in. With a magnifying glass or a strong pair of reading glasses it can readily be deciphered.
On copies the supposed serial number is frequently in the wrong place, for example on the clip, although on the corresponding original it would stand beside the clip or on the cap band. There it is usually barely visible and recognisably not applied by laser. Position, the structure of two letters and seven digits, and the execution as a laser engraving together yield a readily verifiable mark of authenticity.