introduced in 1912, this innovation was rapidly imitated by the other major pen makers. parker introduced the button filler, which had a button hidden beneath a blind cap on the end of the barrel. when pressed, it acted on a pressure bar inside to depress the ink sac. many other firms used a lever set flat along the barrel, which, when pulled, squeezed the rubber sac. it was the rubber sac which made all the difference between the founain pens of the 19th century and those of the 20th century. other firms used a screw mechanism at the top end of the pen to squeeze the rubber sac.