Aha! It's Alan Cartridge.
No, that really was a bit desperate, wasn't it? But knowing each other as we do...
With all those funky fountain filling systems to obsess about, and rightly so, it's easy to forget the charms of the humble international man of mystery cartridge. It's not rock'n'roll, but it's jolly useful.
Aside from dedicated awkwardists like Platinum, Sailor, Lamy and Sheaffer, most fountain pens which take a converter will also take an international cartridge.
But they're not the cheapest solution per millilitre, so why would one bother? Because travel broadens the mind, innit. The rain pours, the wildfires fizzle out and it's obviously summer out there, so it's time to get out and explore - and naturally a decent notebook and proper pen are necessary accompaniments to any expedition. Also, it's jolly tricky manoeuvring a full bottle of ink on the beach.
Fortunately, there are ways to make this everyday necessity swanky, not least with the occasional use of posh packaging. J. Herbin provide a handy tin, for instance, and Visconti have even used classic bakelite for a tube. It's good to travel in style, after all.
Nero has some well-tested international cartridges available from Kaweco, in a handy little box which will slip into a pocket of even the most technically streamlined sportswear. Maybe the screwball rotary dispenser will even join them some day. Summer Purple comes highly recommended, as it happens. Pop one in and get scribbling... wherever you are.