Some of the terms used in describing fountain pen filling systems are less than helpful in understanding how the pens work. For instance, Parker’ Vacumatic and Sheaffer’s Vacuum Filler seem to lay claim to the vacuum principle in filling pens, whereas every pen that has a self-filling system depends on that principle. Whether they be sac fillers, piston fillers, plunger fillers or any other self filling system I’ve forgotten they are provided with a means of creating a vacuum and it is nature’s abhorrence of that vacuum that fills the pen. None is more dependent or makes greater use of the vacuum principle than another. All self-filling pens are vacuum fillers.
“Twist Filler” must be the most misused term of all. In a true twist filler, the sac is attached at both ends, one directly to the turn-button on the end of the barrel. When the button is turned, the sac is wrung out like a dish-cloth. The defining principle of the twist filler, then, is that the sac itself is twisted to empty it of air in preparation for pulling in ink. Almost any pen that has a turn-button on the end of the barrel seems to be called a twist filler nowadays, whereas the majority of them work in another way. Swan’s Leverless is often called twist filler, but in fact the sac is attached at only one end and rather than being twisted it is pressed against the interior of the barrel, thereby expelling most of the air. Various late Mabie Todd pens are called twist fillers when in fact they are most closely related to the button filler. A cam is activated by turning the button, and this presses down on a normal pressure bar which squeezes the sac in the usual way.
Yes, I’m feeling picky and pedantic today.