The model numbers are based on a system that Montblanc started using in the mid-1930s.
Originally, the first digit stood for the line of the pen (1=Meisterstuck highest quality, 3=economy line), the second digit for the filling system or model shape (4=piston filler with round ends, replaced 3= previous piston filler with squared-off ends), and the third digit for the size of the nib, which influenced the size of the overall pen (9=#9 nib size on the pre-1930 safety pens)
If you visit the websites mentioned in pages in the reference thread that's pinned to the top of the forum, you'll see a listing of all these model numbers, and other sites have pictures of many of these vintage models.
http://www.fountainpennetwork.com/forum/in...showtopic=46957