Uni-ball Signo RT1 Gel Ballpoint Pen Blue 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: Blue Black The diameter of a ball: 0.38 mm. Extra-fine type.