The 146 and the 149 are piston fillers: the filling mechanism is built permanently into the barrel, so a converter can neither be fitted nor would it serve any purpose. The 147 Traveller, by contrast, is a cartridge pen that can be run with a converter if preferred. What speaks for the piston filler in heavy use is its far greater ink capacity, which means refilling less often. And anyone wanting to use unusual or coloured inks that are not sold in cartridges will find a piston filler hard to avoid in any case.
What speaks for the cartridge pen is its indifference to changes in air pressure: it leaks less readily on flights, which is what the name Traveller alludes to. The choice is thus less a question of quality than of use.
If a 149 does draw ink when filled but gives it straight back up, leaving ink in the barrel and on your fingers, the piston mechanism is no longer sealing; usually the seal or the piston is worn. This is a common wear item on older pens and is put right by Montblanc or by a workshop specialising in writing instruments.