The basic filling instructions below apply to all plunger-fillers, although most did not incorporate the ink valve feature. In fact, later Onotos dispensed with the ink shutoff, most notably the Onoto Minor. On the other hand, quite a few Japanese eyedropper-fillers used an Onoto-style ink shutoff without any sort of filler piston.
A number of pens were sold full of ink to demonstrate their leakproofness – the Moore Non-Leakable safety comes immediately to mind.
Unlike Sheaffer and Wahl plunger-fillers, Onotos were designed to be serviced without having to drill out old packing units and replace them with original factory spares. Whereas the American ventures into plunger-fillers were relatively short-lived, and the pens are even today widely avoided by collectors due to the difficulty of finding repairmen capable of restoring them to working order, the Onoto enjoyed decades of well-earned popularity worldwide and a continuing following among international pen collectors.
The instruction sheet at left dates to around 1916, while the section detail at right probably dates to the early 1930s. As you can see, the filling mechanism changed little in the intervening years.