Jaxon stud filler (also sold as Conklinette)

The stud is attached to a bar inside the pen. Rotate the metal ring to lock or unlock the stud. When unlocked, press the stud to compress the sac. This pen cites the same patent as the Conklin Crescent. I have to be honest – I don’t think it applies. There is another later patent for a ring that locks a stud, but I have never seen a pen to match it.