For the collector the most easily recognisable part is the small triangular half hooded nib, available in gold, gold plate and steel, which can be unscrewed as a unit with the collar and ink feeder for easy replacement. The size code is usually stamped on the bottom of the nib collar as a single letter or more rarely as a number.
Nib Size Codes
N: Needle Point
X: Extra Fine
BB: Double Broad
C: Fine Italic
D: Medium Italic
E: Broad Italic
FO: Fine Oblique
R: Medium Oblique
Z: Reverse Broad Oblique Italic
Y: Reverse Fine Oblique Italic
J: Reverse Medium Oblique Italic ?
Removing A Nib
1: Unscrew the collar from the shell grip. If you can not unscrew it it is probably stuck with dried ink, do not force it but soak the pen in luke warm water and leave to soak for a while. After this the collar will usually unscrew. Once unscrewed pull out the nib, collar and feed unit from the shell section.
2: To remove the nib from the unit push the collar backwards down the feed. Again do not force it as often on old pens there is a build up of dried ink.
3: The nib should now be free. When reassembling make sure that the 2 prongs at the bottom of the nib line up with the slots on the feed to prevent the nib turning.