Uni-ball Signo RT1 Gel Ballpoint Pen Black Ink Refills, 0.38mm, Set of 4 (Japan import) [Komainu-Dou Original Package] Refill for Uni-ball Signo RT1 UMN-155-38, Uni-ball Signo RT1 UMN-155C-38. Ink color: Black The diameter of a ball: 0.38 mm. extra-fine type.