The K denotes the nib width 'Kugelfeder' (ball nib). Such ball nibs were also offered by other makers like Pelikan or Faber-Castell as KF, KM etc. and often have two breather holes, which allows stronger turning in different directions — good for changing writing posture, e.g. in exams. In writing behaviour they count, however, as rather characterless. A detailed overview of rare nib widths at Montblanc can be found in the fountainpen.de newsletter from February 2005 from page 12.