A recommendation, but a warning to go with it. And, these probably aren't the pens you are referring to.
You can try Kohinoor Rapidograph technical pens. I used them for years on drafting boards, and they are very high quality. You can get them with several tip styles, steel, jewel, and synthetic I think. The line styles vary from 000 which is about 1/4mm I think up to about 4 or 5mm. It's been a long time since I used them though. The drawback is that they are expensive. The handles are cheap but the tips are $20+ each.
You can also try a 2-nib drafting pen. This one is inexpensive, and has two nibs held apart by spring tension. The width of the line is varied by opening or closing the nibs.
The problem with these is that the tips are hard. Especially the 2-nib pens. They are steel and you are going to be dragging it across your paint job. If you are careful and use very little pressure it may not cause any problems, but I've never tried it on a model.
Like I said, I don't think these are the ones you were talking about, but thought I'd throw the info in anyway.