And why not? The principle was the same as that of a bulb-filler or Vacumatic, with additional benefits: the transparent plunger allowed the ink level to be viewed, and the rubber sac acted as both reservoir and spring. The only problem for us now is finding replacement sacs, which are long out of production and unavailable. I am confident that we will have a source for replacements within the next few years; I am working on this myself, and others are too. In the meantime, though, what is to be done with older French pens whose original accordion sacs are no longer usable?
Some have turned them into bulb-fillers, by discarding the transparent plunger and installing a conventional sac of sufficient length to stick out under the blind cap where the plunger used to be. This works, but is decidedly less than elegant -- and doesn't allow a converted pen to be re-converted to its original configuration once accordion sacs become available once again.
Others have retained the plunger and used a length of rubber tubing cut from a regular sac. This works, though imperfectly. The sac doesn't collapse evenly when compressed axially, making the plunger action jerky and reducing the filling efficiency.
Nonetheless, with a bit of a twist, this is the best option currently available. The twist being to treat the modified pen as a pump-twist-filler, rather than as a plunger-pump. That is, instead of pressing the plunger, one twists it and releases it several times, until the pen is full.
There are some pens for which this will not work, however. The full-size and oversize Stylomines, in particular, have a metal cage around the plunger that prevents one from giving it the requisite twist.
Possible solutions would include attaching the cage to the plunger, permitting the entire assembly to be twisted, as well as removal of the cage -- which should be retained, pending availability of original-style sacs. And of course, one could always unscrew the section from the barrel and twist away, putting the barrel back in place after filling. As it happens, however, I may already have found an off-the-shelf item that can be used as an accordion sac for the largest Stylomines, and as noted above, it is only a matter of time before sacs in all sizes are available once again. So please, whatever you do, don't throw away any original parts!