As the thirties progressed and the depression hit, companies began to look for ways of making pens which were (preferably both) cheaper and more technologically advanced than their competitors.
Generally these developments offered little significant advantages over their predecessors, being launched primarily for fashion or marketing purposes.
It was felt, for example that an ink shut-off was needed to reduce the possibility of ink leakage, especially in conditions such as those found on aeroplanes; this ignored the fact that the recently discontinued safety pens made by Watermans , Moore and Montblanc (amongst others) contained a sealed ink chamber and therefore couldn't leak even under extreme conditions.
Thus complex ink shut-off mechanisms were developed and offered as advances over the competition. They didn't work and soon the companies producing them were forced to stop making the claims that they did.
Meanwhile safety pens were continued in production; in fact Watermans had to re-introduce safety pens specifically for aircraft use. Watermans of France had always been sceptical with these new-fangled technological advances and had always been selling safety pens, so it was relatively easy merely to rename these old designs Aero-Watermans, -presumably now hailing them as new technological marvels of modern science.
It was thought to be an advantage to have a transparent part of the barrel through which the remaining ink level could be seen. The difficulty was that one couldn't easily put a transparent section of the barrel near a rubber sack; If you looked through a transparent barrel all that could be seen was a black sack, and no one had yet developed a transparent or translucent sack.
Sheaffer and Eversharp went to great technological lengths to put transparent sections between the section (behind the nib) and the barrel threads to show remaining ink levels, ignoring the crude developmental levels of the plastics involved, which were simply not up to the level of the ingenuity of the pen designers.
Parker adopted a different tack with their Vacumatic, replacing the sack with a rubber diaphragm at the end of the barrel and introducing complex mechanisms for holding the diaphragm in place and utilising it to suck up ink.
Eventually most of these mechanisms had to be discarded and the companies went back to 'old' systems, touted of course as radically different technological breakthroughs.
Initially Aurora in Italy found that they had to re-introduce the eye-dropper filler during the late thirties, ostensibly solely as a convenience to italian soldiers who needed a reliable 'trench' type pen, as developed by Watermans during the first world war.
This was a pen which was an eyedropper filler, but with a compartment in the barrel end which contained ink pellets. These pellets were inserted into the barrel, and the barrel was filled with water, which thereby 'became' ink. It is however clear to collectors who have seen these pens that they were not intended for the exclusive use of soldiers in the Ethiopian campaign (they are all in a beautiful white colour with an italian crest and ETIOPIA inscribed on the barrel), and these pens are occasionally found anywhere Aurora sold their pens during the thirties.
Although Parker pushed their Vacumatic pens from around 1932, they did not discontinue their duofold button filler mechanism until the aerometric filler was firmly in place , -after the demise of the Vacumatic filler which it was designed to replace; they introduced their aerometric filler in the fifties on their 51 line, and it bore more than a passing resemblance to the old sleeve filler idea used by Watermans briefly during the first world war.
Having been forced by wartime shortages to discard most of their complex thirties technological 'marvels', Watermans and Swan discovered that the lever (or similar) pushing a pressure bar depressing the sack was not such a bad idea after all, and continued it into the middle of the century.
When Swan tried to make their pressure bar system more complex in the fifties by introducing a button at the end of the barrel to depress the pressure bar similar to (but more complex than) the old Parker Duofold button filler of the first world war era, they went broke, and when Watermans decided to go down market with their products after the forties, they too in America went broke. When Watermans were rescued by their french subsidiary, they (relatively soon) adopted the old french practice of not having a filling mechanism at all, merely a removable cartridge in the barrel, this time made of transparent plastic.