A pen to hold a standard gel ink rollerball cartridge, designed to replace a defunct fountain pen assembly in a pen I got on vacationWhen I was on vacation, I got a really nice looking fountain pen. Unfortunately, the innards were junk - every time I put a new ink cartridge in it, it would dry out and die.The solution was to design a custom pen tip that screwed into the barrel the same way, had the same retaining collar to hold the pen cap on - but instead housed a standard gel-ink rollerball cartridge.