The fountain pen is most likely genuine; heart-hole nib, feed and barrel imprint speak for a manufacture towards the end of the 1920s. The cap seems somewhat shortened, and also the lever appears not to be original, since the correct lever had the Montblanc star engraved on the round lever end. Regarding the stated size 6, it is a comparatively rare model. A definitive determination requires further detail photos, especially of the barrel end and any further engravings.