The Pilot CON-40 fountain pen converter fits most Pilot cartridge converter style fountain pens. Pilot pens use a proprietary ink cartridge and can only use Pilot converters as well.
Each Pilot CON-40 converter comes with a clear plastic body to see the ink level. Inside are small ball bearings to agitate and keep the ink fresh. The converter has a small twist knob to control the piston within the converter to fill with fountain pen ink.