The Pilot Converter 70 is compatible with some of the higher-end Pilot fountain pens. The following Pilot pens advertise compatibility with the Converter 70: Custom 74 series, Custom Heritage 91, Custom 743, Custom Heritage 912, Custom 845, JustUs 95, Elabo, and the Maki-e series. Even though Pilot does not advertise this converter as compatible with the popular Kakuno fountain pen, we use this inside our Kakuno and it works just great! This is a frequently asked question lately, so we wanted to share our confirmation that it does work. We love this compatibility because it means we can hold twice as much ink in our Kakuno as we could previously! Plus, the pumping mechanism on this converter is fun to use!