No. 334 1/2
The 334½ was produced before, during, and a little after WWII. This one in my collection is from 1947-52 since it has two gold rings on its cap and has a 14k gold nib inscribed with 4½ to denote that it is the correct size for this pen. It is interesting to note that the catalogue of the 1930s suggested that the 3xx series could be exchanged within a year for the "Meisterstück" series or 13x series for simply the difference in price between the pens. As per Montblanc Diary, however, only 5% of the customers actually exercised this option!
I suspect that this pen was replaced the button filling 324 which is really quite similar. One key difference from the 324 is that the 334½ is a piston filler. As you can see to the right, the pen was available in black, blue, green, and other colours.
It is so interesting that MB felt the need to have a ½ size in the lineup. To me, this makes the pen rather special. There was a 333½ made as well.
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