Most collectors put vintage Watermans into two basic categories, eyedropper fillers and lever fillers. In fact, Waterman used no less than four different types of self-filling mechanisms prior to adopting the lever-filler in 1915 (not 1913, as some sources mistakenly state).
All four mechanisms are rare and of considerable historical and technical interest. The first, a syringe-filler using a piston attached to a long shaft hidden under a long, tapered blind cap, dates to the early 1890s, with the few surviving examples datable to c. 1895. No catalog listings, advertisements, or illustrations are known.
The second filler was a pump mechanism, illustrated above. The end of the barrel was unscrewed and then gently oscillated. The mechanism was described in two patents, issued in 1897 and 1900, but it appears as if production did not begin in earnest until around 1903. These pens are rare, and though they were not consistently featured in advertisements and catalogs, this is as likely to be a result of their lack of popularity rather than the cause. Although some state that the pump-filler was dropped around 1910, when the sleeve-filler was introduced, scattered listings appeared as late as the early 1920s, which has led to speculation that this was done to keep up patent rights.
The third mechanism was the sleeve-filler, in which a sliding sleeve covered a cutout in the barrel, through which one could depress a pressure bar resting on the ink sac. Sleeve-fillers are scarce, but not nearly as rare as the other early self-fillers. Note that nonoriginal replacement caps are often found on these pens: slip-caps from eyedroppers are longer than proper sleeve-filler caps and usually slightly smaller in outside diameter; screw-caps are somewhat closer in overall proportion, but have inner caps and internal threads which are absent on the original slip-caps. The patent (US 950817) for Waterman's sleeve-filler mechanism was applied for in August of 1909, with the patent issued in March 1910. Production appears to have begun somewhere between those two dates, with mention of both self-filling and pump styles appearing in the February 19, 1910 issue of The American Stationer (vol. 67, p. 6). Sleeve-filler production continued until the introduction of Waterman's first lever-filler in 1915. A variant with a rotating rather than a sliding sleeve was patented in 1905 as US 799897. This design was never used for Waterman's top-line, but was promptly put into production under the "Remex" sub-brand. It was also used by Aikin Lambert after its takeover by Waterman around 1907.
The fourth mechanism was the coin filler, in which a simple slot was cut into the side of the barrel, enabling the pressure bar to be depressed by insertion of a coin (Waterman also made a special oval "coin" to go with these pens). Although their rarity and the almost complete absence of advertising has led to the belief that they were offered for only a very short time, recent research indicates that Waterman coin-fillers were produced from c. 1910-15 -- much the same span as the sleeve-filler. Waterman did not invent the coin-filler, which was a popular design among contemporary makers of economy-line pens, and by all appearances never patented. Note that there are a number of fake overlay coin-fillers now in circulation made from screw-cap filigree eyedroppers, some of which came with overlays originally intended for coin-fillers. The proportions of these fakes are usually somewhat off, but one has to have experience to detect this. More obvious discrepancies include incorrect numbers on the end of the barrel (POC instead of PSF), badly centered coin slots, and incorrect interior parts (J-shaped springs instead of the original blackened and grooved unsprung pressure bar). Note that coin-fillers had screw caps, whereas all the earlier self-fillers had slip caps.
more on the Waterman numbering system