M200 and M250 are identical in housing and differ only in the nib; both consist of the 'cheaper' plastic with weakly gilded applications, while from the M300/M400 higher-quality materials are used. Theoretically the 250 and 400 carry the same feed; in practice over the decades, however, different feeds were fitted. When transferring a 250 nib into a 400 housing only the nib without the old feed should therefore be taken over, since the old screw-thread feeds, despite similar construction, do not reliably hold in newer housings. Transferring into 200 or 250 housings is also possible by the same rule; in doubt an experienced specialist helps.