The activated charcoal filters by Butz Choquin are made to fit their 9mm smoking pipes. These filters also work well for Vauen Pipes and some Castlford pipes. This type of filter is popular in the European market because it reduces nicotine and tar, providing the smoker with a cooler, lighter and less wet smoking experience.
(Just a note these will not work in a Boswell Pipe As J.M. does not make them with to take filters.)