This is the UMR-38S black model refill for the Mitsubishi Uni-ball One gel pen. With a 0.38mm ball diameter, it provides a precise and vivid writing experience and can be easily replaced when the ink in your existing Uni-ball One retractable pen runs out. These are genuine refills manufactured in Japan, available individually for economical purchasing. This product is for refills only and does not support printing or engraving; please verify compatibility with your existing pen body before purchasing.