Montblanc built the Carrera both as a cartridge pen and as a piston filler, so both versions are original. The cartridge pen is by far the more common, and supposed piston fillers usually turn out to be precisely that, with a piston converter fitted.
The two can be told apart with a glance inside the barrel. A converter sits loose where the cartridge would otherwise go and can be pulled off the nipple, in some cases with a certain amount of force; the pen can then be filled with standard cartridges as normal. A true piston filler, by contrast, has the filling mechanism built permanently into the barrel and is operated by a knob at the end of the barrel which cannot be removed. The tightness of the converter puts many owners off, but it is nothing to worry about as long as it is pulled straight and not at an angle.