Montblanc numbering system
In the late 1930’s Montblanc introduced their new numbering system for pen models. This consisted of a three-digit number, sometimes with an additional letter at the end. This model number can be “read” if you know how to decode it. The third number is the size of the nib.
from https://www.pm-pens.com/2018/06/16/montblanc-numbering-system/
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