I'm pleasantly surprised this isn't the community for fountain pens like I expected.

You could make a second part that's always screwed into the screwy bit and then remix the new cap to pop into that!

The other commenter is correct, this is a calligraphy pen.

The parallel plates of the nib are flat in cross section which isn't very clear in the picture I took, which had the pen just oriented randomly. Maybe I should re-take it with the flat side towards the camera.

The Parallel takes the normal fountain pen nib design with the split down the middle and kind of flips it on its head. The ink flows between the two plates and because of this you get very even coverage. The Parallel is capable of, if you keep up your end of the operation anyway, producing really sharp, very square, and highly directionally differential lines. Even moreso than a traditional fountain pen. It's also not as prone to having its ink dry out over short periods of disuse, at least in my experience.

Of course you can get them in an array of nib widths and of course I have all of the OG sizes. A 3.0 and 4.5mm nib were released later after I collected my whole set and one of these days I'll get them, too.

They're mostly used for calligraphy.

The original caps look like this. There is a fin on it that prevents the pen from rolling away -- only when it's screwed on the business end.

As you can see this is not actually a pocket clip, and also the cap cannot be "posted," that is, stuck to the tail end of the pen body. You can balance it there, but it doesn't lock or or grip so it won't stay there and just falls off. So you can't carry the pen in your pocket without it becoming annoying (although it is quite long, sized and shaped more like a dip pen nib holder, so make sure you have deep pockets) and you can't mount the stock cap on the tail to keep the pen from rolling away if you're using it on a sloped surface like e.g. a drawing or drafting table.