All of these early pens dating from the 1880s to the mid nineties used an eye-dropper filling mechanism.
At the turn of the century, attention turned to refining the filling and the reservoir system, with two basic paths being followed.
One used the barrel as a reservoir, concentrating on a piston to create a vacuum inside it, which fills the barrel with ink when the vacuum is released into an ink bottle.
The other used a rubber sack to contain the ink inside the barrel and some form of pressure bar to depress the sack while the nib is immersed in ink, thereby drawing ink into the sack. This method of filling was first commercially applied by Conklin in Toledo with their famous Crescent Filler. The crescent protruded from the side of the barrel and a pressure bar was attached to the it's underside inside the barrel. A locking ring was rolled under the crescent to stop the pressure bar pushing against the sack when the pen was not being filled.
This method was copied all over the world (initially by Parker with their click-filler and by Wirt in their locking "hump", then by A. Nu¤es, Spohr etc), even after it had been dropped by Conklin.
A half-way house was the matchstick filler developed (probably) by Weidlich. This used a hole in the side of the barrel through which the used pushed the pressure bar with a matchstick. In the Weidlich, non-smokers were provided with a tremendously inelegant matchstick sticking out of the end of the pen.
The idea was resuscitated
during the thirties by the Zerolo which had two separate nibs, sections
and sack chambers within the barrel for either two colours or, less commonly,
two types of nib. One side spiralled out while the other spiralled back
into the barrel as the end was turned (much like the safety pen).
As there was no space within the barrel for levers or filling mechanisms, a simple hole was put into the side of the barrel to push the pressure bar which was integral with the sack-chamber.
The matchstick unscrews from under the blind cap above the clip. Because of the complexity of the mechanism along with the difficulty of synchronising the two nib sections to come out at exactly the right time; and be synchronised with the hole for the matchstick to marry with the pressure bar, it is not recommended that any novice try to replace the sacks in a Zerolo.
Anyway to return to the mid-teens, both Watermans and Parker were experiencing considerable difficulties in competing with Sheaffer's lever filler.
Watermans and Parker were still touting the eye-dropper at the beginning of the teens and were rapidly losing sales to Sheaffer so they had to find their own self-filing mechanism.
Parker used the button filler, in which a button under a blind cap at the end of the barrel exerts longitudinal pressure on a sprung pressure bar within the barrel, which in turn springs out into the sack . This was the method used by Parker for their famous Duofold range, so called because originally the blind cap was interchangeable with a long thin continuation of the barrel, converting the pocket pen into a desk pen.
L.E. Waterman tried several different ideas: the coin filler (a matchstick-type filler which used a coin); the sleeve filler (probably developed initially by Century or Aikin Lambert) in which a sleeve moved along the barrel revealing the pressure bar which could be depressed with a finger; the pump filler before eventually alighting on their own lever (within a box section) which apparently did not infringe Sheaffer's patents.
The W.A. Sheaffer Pen Company of Fort Madison, Iowa had developed the familiar lever which fitted flush with the barrel when not in use , and this became the most used mechanism for the next forty years. There was however also in use at that time the button filler mechanism