Caps and barrels of Montblanc pens from the 1920s to the 1940s, the model 334 for example, are as a rule made of hard rubber. The material is deep black to begin with, but oxidises slowly under the influence of air and light and takes on a brown to olive-green tone in the process. The discoloration goes deep and cannot simply be polished away.
Aggressive polishes and solvent-bearing polishing wadding attack hard rubber further: the colour shifts towards yellow-green and the material then smells sharply of plastic. First aid is to rinse the chemical off with water, but only briefly, since prolonged contact with water also promotes oxidation. Soaking for hours does more harm than good. On metal parts, a propelling pencil with an electroplated surface for instance, the same polishing wadding is usually harmless, which readily leads to false expectations.
For hard rubber and other historic materials a mild polish such as Wenol is regarded as the right choice; collectors report that it is used at Montblanc itself. Polishes for acrylic, Displex among them, suit only current pens in precious resin and will destroy old surfaces. Many collectors deliberately leave well alone in any case, because an even patina is part of the character of an old pen, and a body lightened by polishing will darken again in time regardless.