A converter is the part that lets a cartridge pen use bottled ink: a small piston filler that plugs into the same nipple a cartridge does. This one is a standard international fit, 0.75 ml capacity, with a brass piston nut rather than a moulded plastic one and a loose steel mixing ball inside the barrel.
Both of those are the difference between a converter that lasts and one that does not. A plastic nut strips if the piston is ever forced against a dry seal; brass does not. The ball stops ink separating and, more usefully, it knocks ink off the barrel walls so you can actually see the level — without it a converter looks full when it is a third empty.
Fits every pen in this catalogue except the Quire Pocket Pen, which has no room. Wash before first use. The piston seal is silicone and can be re-greased with a trace of silicone grease every year or two.
