#sorry for ugly unclickable links - I'm writing from mobile Firefox because I refuse to install the app

[slightly-sigilant]

I'm finally digging into my ES backlog, starting with Shades of Yesterday. I can tell that the writer's actually familiar with fountain pens and it delights me tremendously

Everything in this passage is a real fountain pen filling mechanism, as far as I can tell:

"filling your pen with an eyedropper" - eyedropper fillers, exactly what they sound like. And yep, some do have an unfortunate tendency to leak through their screwed-together bits.

"a rubber sac inside the barrel" - squeeze sac fillers, where the ink container is a little flexible sac you can squish and unsquish, sucking up ink to refill the pen. I think this type was more common in the past and has mostly been superseded by piston and vacuum fillers, but they still exist (Pilot Metropolitans come with a squeeze-style converter, for example)

"the manipulation of air pressure to suck ink into the pen" - I think this one's referring to vacuum fillers but I'm not 100% certain?? It's the kind of filling style I have the least familiarity with.

"refills with a simple twist of the base" - piston filler, maybe?

That being said, it's possible that all of these lines are just describing variations of sac-based filling systems, since Fallen London is set in the late 1800s and (according to Wikipedia and what I can scrape together from a bunch of fountain pen nerdery sites) all of the early fountain pen non-eyedropper filling designs were based around sacs rather than the plastic tubes that most piston/vacuum-filler converters use today. The mechanisms for deflating the sac varied, but the principle was the same - squish sac, put nib of pen in ink, unsquish sac to pull ink into the pen.

Conklin crescent-filler: https://rupertarzeian.com/2023/04/08/early-thoughts-on-the-conklin-mark-twain-crescent-filler-fountain-pen/

Waterman twist-filler: https://vintagepens.com/filling_instructions_twist-fillers.shtml

Sheaffer lever-filler: https://www.newpentrace.net/article102501_127.html

Parker button-filler: https://penhero.com/PenGallery/Parker/ParkerDuofoldFiller.htm

Full disclaimer that I'm not a pen expert or historian so I could be wrong about some of these