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 first number is the price category
1: Meisterstück or Masterpiece
2: Medium priced
3: Economy Model
The second number indicated the filling system
The third number is the size of the nib.
Occasionally, a letter would follow, indicating the finish and colour
E: Chased rose pattern
PL: Silver grey / Black
This info excerpted from Fountain Pens of the World, by Andreas Lambrou, published by Classic Pens Ltd. If you are interested in fountain pens, you should own this book!