The model described is likely to be the grey marbled M200; replacement nibs for this are easily available from specialist retailers or from specialists like Rolf Thiel (missing-pen). Small worn spots on the nib usually arise mechanically, e.g. from frequent use with a steel ruler; a localised rust spot is regarded as very unusual. Among the Souveräns, M405, M605, M625 and M805 differ mainly in size and price, with the M405 being too small for many, while the M805 is very well manageable. Since the nibs are hand-ground, an extensive test write in the shop with one's own paper is strongly recommended.