JUST in case anyone is interested... I ordered a couple of the cheapie china 3.5 ml DC tanks (as opposed to the 6.5 ml versions everyone is snapping up) to see how they'd work on the Darwin. These tanks are only 15 mm in diameter.
Turns out that with a 1 inch extender, it works pretty well. Not a LOT of extra capacity, but at least a good visual indication of nicquid level, and the arm can be closed for easier pocketing.
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Since this is the closest I've found to a closable tank that happens to be actually available, guess it'll have to do!
Brandon showed a new elbow that might go into production to allow tanks usage more easily on tonight's VT Talk at VTTalk with Guest Brandon Ward

Enjoy!!!
Drew you working on this at all ??? or is it just going to be the bigger Arm ? people with the smaller arm will still need and want this 'S" I think Drew. Like to here your thoughts on this when ya get a chance please.