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Therefore, the bottoms of classic piston fillers are classifiable as such (not those used to protect a knob underneath), the knobs of plunger filler system, those of safetiea and also those of some specific types of button fillers (like the push-knob). In all of these cases the knob is a constituent part of the pen's filling mechanism, and is used for handling. A photo gallery can be found on this page.