This small eraser refill can be slipped into your Kaweco Special Mechanical Pencil when the eraser has been rubbed bare with use. 

eraser within metal case 
1/2"

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.