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Cobra ring. Size US 4. Adjustable 15 mm +/- 1 mm *Please contact me for other sizes.
For the roll-stopper use: Please adjust the stopper size slightly smaller than the pen size and attach to the pen cap or barrel.
Warning! Please use this product at your own risk. Since you are ordering it directly from Shapeways, and attaching it by yourself, I will not take any responsibility if your pen gets damaged by your actions.