Epenco Plunger or Frankie

I got this pen a while back and finally got around to cleaning it up.  It is an Epenco, but unlike any I have seen.  It has the long breather tube, but not the filler mechanism I have seen on other Epenco (the little lever).  It does have a bit of an old sac. Did this pen just have a sac that was squeezed by fingers?