The number 84 is a Montblanc designation that really was sold as such and no mark of a fake, even if the neighbouring 82 is far better known. The Montblanc push-button filler with the number 15, from the early 1920s, likewise existed.
An imprint reading “KX 125”, by contrast, is not documented and probably rests on a misreading: the candidates are a size marking such as 12.5 or, more likely, the model number 126. The lettering itself is a further aid to dating, since the name Meisterstück was only stamped on from 1924 onwards. A pen carrying that imprint therefore cannot date from 1921, as is occasionally claimed of such pieces. The early Montblanc numbers are on the whole only patchily documented, so any attribution should always be checked against the nib, the filling system and the design of the cap as well.