Pilot Petit1s. ...
Not sure if there's a converter that fits these little things, so I plan on refilling the cartridges with a syringe.
I have a couple that I got from Jetpens ... they are good pens, as you say. I use mine as "eyedropper" pens and they work well for that (as do the Kaweco ... just don't "eyedropper" a metal pen!). That really increases the ink capacity!