James, I did not buy it, I made it: I turned a 4mm OD piece of iron wood, drilled it and then glued a 2.5mm mother-of-pearl dot at one end, rounded it, and glued it in a blind hole I made in the cap.
Btw, did I tell you I can fit my pens with oblique nibs, and I can grind them myself to fit user's special wishes? :stung: