The M engraved on the nib stands for Montblanc and not for the line width; the actual nib width is communicated by the works only by sticker to the dealer. Without this sticker the determination is only possible under the loupe and with experience, with at most more extreme grinds (OB, OBB) being well distinguishable from M, B or BB. On eBay the stickers are often still present, so the search for an M nib in practice is feasible.