The Pilot CON-B is an aerometric, or pump-style, fountain pen converter.
The basic construction of the converter — as is the case with most aerometric converters — is a rubber bladder inside of a metal shell with a press plate on the side. You’d submerge the bottom of the converter in ink, press the plate and the ink would bubble. After you would let go and the spring-loaded press plate would suck ink into the bladder making for a cheap, but not a very good converter.
The capacity of this converter is about 0.6 ml, which is in line with that of the CON-20 and Pilot’s cartridges.
This converter is often confused with the more desirable and more expensive CON-20. That converter was discontinued in 2017 and replaced by the CON-40.