...because it was just being held onto the sac nipple with a small O-ring. Mine was one of the 1st generation modern Conklin Crescent-fillers, but I think the next generation had the sac glued on. A friend of mine had one of those and the sac on his Crescent-filler melted so he had to remove the sticky remnants and I installed a new sac on it. The pen itself is a nice size to use--there's no posting option-- and some of the acrylics are quite pretty I'll keep the Pilot/Namiki cartridge option in mind . Thanks for sharing!