De La Rue's Onotos were plunger-fillers that incorporated an ink shutoff. If the plunger handle was screwed down all the way, a cone at the end of the plunger shaft closed off flow to the feed. Many Japanese makers made this shutoff a feature of their eyedroppers.
The only author to have pointed this out is Jonathan Steinberg; too many collectors seem to have missed this nugget while focusing on the book's illustrations, because we are often asked about filler repair on these plunger-fillers that aren't!
NOTE: De La Rue themselves made eyedroppers with ink shutoffs that closely resembled their regular plunger-fillers. The Golden Swift, for example, was made for export to France.