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Wraps around a Pilot G-2 Pen cartridge, requires gluing in it's current iteration (So don't expect your cartridge back. They're like a buck a piece though.)
Now obviously there's nothing wrong with the G-2's body, this one is just nicer looking, smaller, and comes with a certain amount of bragging rights ("Was *your* pen made in a 3D printer?")
Warning: As of this writing I haven't gotten one yet, so I can't guarantee anything about how well it'll work. It probably needs pretty strong material though, since the walls are often less than a millimeter thick. Also, this is still a proof of concept more than anything else, so it doesn't have any sort of normal pen moving-parts just yet, although I'd be interested to see whether or not that's possible with 3D printing later.