A mini piston-filled converter to use with smaller fountain pens like the Kaweco Sport. This allows the pen to use bottled ink rather than cartridges.
Kaweco was established in 1883 outside Heidelberg Germany, although it was not yet called Kaweco and the design that is the most recognizable, Kaweco Sport, was not designed until 1911. A fountain pen that was not a heavy, expensive desk piece but rather lightweight and convenient was an innovation. Ballpoint pens, what we would think of as an analogy to this experience, were invented in 1888 but a reasonably functional design was not produced until 1938 and popularization came mostly after World War II, by then the Kaweco Sport was a classic as well as many other designs produced by Kaweco.