This is one Blue Black 0.38 mm UMR-83 gel ink refill. The tip is rounded and edgeless to reduce friction against the paper, resulting in a super-smooth writing experience. It is compatible with the pens listed under Compatible Products.
This is my favorite refill because Blue Black is different yet still classy. The
ink writes incredibly smoothly and it fits the barrel of the pen very nicely as
well. I buy this in bulk because it can run out pretty quickly if this is the
only pen you use every day.