The Meisterstück Le Grand 146 is without exception a piston-filling pen. You turn the piston knob at the rear end, the piston travels down, the nib is dipped in the ink, and winding back fills the barrel. A converter is neither necessary nor suitable for this; there is no Le Grand version into which a converter unit could be fitted. The 147 Traveller in the same range is, conversely, a pure cartridge pen, and no converter fits that either. If the barrel of a 146 visibly ends at a thread and the mechanism is missing altogether or has broken off, that is a fault, not a missing accessory.
As a matter of policy Montblanc does not supply spare parts to customers, partly to prevent complete pens being assembled from components. A damaged or missing piston mechanism can therefore only be replaced through the factory's customer service or through a workshop specialising in Montblanc. The sensible course is to send the pen in and have an estimate drawn up first. Replacing a piston mechanism is a routine job and has in practice cost a good deal less than the figures of several hundred euros sometimes mentioned; only with the estimate in hand can you decide whether repair makes sense in relation to the market value.