What does the addition N after the model number of a Montblanc Meisterstück mean, e.g. on the 644 N?

The appended N marks on older Meisterstücks the version with wing nib. Models without N are, by contrast, equipped with the classic Meisterstück nib. The distinction is well comprehensible based on the nib illustrations in relevant overviews (e.g. at Maxpens or Penboard).