Mutsu no Kami , the third-generation head of the Hashimoto main line in Province, was born in 'ei 14 (1637) as the legitimate son of Omi no Daijo Tadahiro. Known by the common name Shinzaburo, he succeeded to the name "" after the Tosa no Kami branch returned the right to use it to the main house. In the tenth month of Manji 3 (1660), at the age of twenty-four, he received the court title Mutsu no Daijo, and in the eighth month of the following year, 1 (1661), he advanced to Mutsu no Kami. He died in Jokyo 3 (1686) at the age of fifty, predeceasing his father by seven years. The consistently observes that the relative scarcity of his extant works is due both to his comparatively short working career and to the fact that he served as a substitute maker on behalf of his father. In both construction and manner of workmanship, his style is closer to that of his grandfather, the first-generation , than to that of his father, possessing a bold, vigorous, and unconstrained character that the describe as powerful and spirited.
The third-generation is recognized above all as a master of . His signature displays deep , thickly adhering , fine and , and a that is invariably described as bright and clear (). His is a hallmark of the school: tightly forged with abundant that produces the so-called — a distinctive rice-bran surface texture — through which fine enter. The bosshi characteristically runs straight, turning back in , with the tip often showing a tendency toward . His blades tend toward imposing construction: long in , with comparatively wide , thick , and deep , presenting a bold and powerful . While was his greatest strength, the also notes that he produced accomplished , including mixed with , , and rich in variation. His is described as especially strong — even stronger than his grandfather's — and many remark that his forging is tighter and clearer than that of the first and second generations.
The situates the third-generation as the member of the main line whose work most closely resembles the founding generation, an artisan whose workmanship is "unmistakably highly skilled, yet possesses a spirited vigor." His finest blades are praised as works that "fully manifest his true abilities" and represent "the finest workmanship of the third-generation ." Dated examples are noted as extremely rare and therefore especially valuable as reference material for the study of swords — a blade bearing a 2 (1662) date is singled out for its contribution to clarifying chronological changes in his signature style. One example, with its and described as — sound and well-preserved — is recognized as a masterwork among his productions, while another was transmitted in the Nabeshima family, underscoring the esteem in which his work was held within the domain he served. Across the body of designated works, the recurring assessment is consistent: a smith of bold and orthodox manner whose best pieces display, without reservation, the full capability of his art.