Kunimasa (国正) was a smith within the circle of Kunihiro, active during the early period. Sources in the variously describe him as either a younger brother of Kunihiro or the father of Osumi no Jo Masahiro; in either case, his position within the ichimon is firmly established. During Kunihiro's lifetime, Kunimasa is believed to have served as a — a substitute maker working on Kunihiro's behalf — a conjecture supported by the extreme rarity of his signed works and by the close resemblance of his workmanship and shaping to those of Kunihiro himself. Among his few extant signed pieces, examples bearing the inscription "Bushu ju Kunimasa" indicate that after Kunihiro's death he relocated to . There is also a general tendency for him to be confused with Hojoji Kunimasa.
The forging of Kunimasa's blades is unquestionably that of the group. The characteristically shows with standing grain and a bold, coarse () texture; forms and enters. The typically presents mixed with , at times incorporating arrow-notch ()-like elements and angular turns; adheres along the temper, runs through, and appears. The may exhibit a tendency or, conversely, appear clear and bright (). The is tempered deeply, returning in with , or in coming to a point. These features well demonstrate the characteristic traits of work.
Kunimasa is a figure of particular significance for the study of the school's workshop practices. The correspondence between his workmanship and that of signed Kunihiro pieces — extending to the precise manner of shaping the — provides material evidence for the system by which the atelier operated. His finest signed works stand out as ("white eyebrow," meaning the finest) among the production of the disciples, and by their existence one can fully appraise Kunimasa's technical worth as a powerful collaborator within this influential -era school.