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I don't recall where I acquired this thing:

5-1/4 inches overall length. Here's the clip:

And the barrel joint (lower barrel fits inside the upper, but fits over and external thread on the mechanism inside):

Monteverde D1 refills in it now, replacing the dry Fend refills it came with. Here are the button and clip:

That ring unscrews from the mechanism inside, releasing the clip. Under the tip of the clip is a slot into which a little tab fits from inside. Pressing that tab in allows the entire mechanism to come out; it's very well-made!

And here's the top of the button:

The closest I've been able to find to this gravity-select pen is a Rotring that's nearly identical in design, though the lower barrel is chrome-plated, the clip is different, and the top of the button has the red ring on it.

I've tagged it as a Fend product, simply because of the refills it came with, but I'd love to know who made it; there are no markings on it whatsoever. The clip is similar to some Sheaffer and Pilot clips, and a little less so to a Waterman design. The lines on the black part look to be cut rather than molded in.