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, can be dated to the very end of 1890s.  Only two or three examples have come to light, and no catalog listings or illustrations are known. 

The second filler was a pump mechanism, illustrated above.  The end of the barrel was unscrewed and then gently oscillated.  These pens are very rare, though advertised and catalogued; they were offered from 1903 up until c. 1910, when the sleeve-filler was introduced -- though strangely enough, scattered listings appeared as late as the early 1920s, which has led to speculation that this was primarily to keep up patent rights. 

The third mechanism was the sleeve-filler, in which a sliding sleeve covered a cutout in the barrel, by 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.  Sleeve-fillers were made from c. 1910-15. 

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).  Waterman coin-fillers were produced for a brief period c. 1913-14, and are very rare today.  Note that there are a number of fake overlay coin-fillers now on the market made from screw-cap filigree eyedroppers.  The proportions of these fakes is 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 used a screw cap, whereas all the earlier self-fillers used cone-cap slip caps.

more on the Waterman numbering system

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