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The family
The Japanese Sword Index relates that, among the branches of the family of the first Tadayoshi (忠吉, Tadayoshi), those of Masahiro, Yukihiro and Tadakuni were prolific and worked around Saga for the Nabeshima and their relatives. The Sanada Museum in Nagano presents the first Hizen Tadakuni as a swordsmith in the service (御抱, okakae) of the Ogi domain.
His name
The 日本人名辞典 (Nihon jinmei jiten) of 芳賀矢一 (Haga Yaichi), in its entry for the first 藤原忠国 (Fujiwara Tadakuni), gives him the surname 橋本 (Hashimoto), the title of 播磨大掾 (Harima Daijō) and the monastic name 播磨入道休鉄 (Harima Nyūdō Kyūtetsu), at 肥前小城 (Hizen Ogi). It has another entry for the third, who was also Harima Daijō. The wazamono list of Japanese Wikipedia records him as 播磨大掾忠国 (Harima Daijō Tadakuni), of the first generation, with the signatures 肥前住播磨大掾藤原忠国 (Hizen jū Harima Daijō Fujiwara Tadakuni) and 肥前忠国 (Hizen Tadakuni), and refers to Sesko (2012).
His dates
Sesko reproduces a wakizashi signed Harima Daijō Fujiwara Tadakuni and dated 1636, and another with the same signature without a date. The year in which he received the title, that of his birth and that of his death are not recorded in any of the sources consulted.
Who he is confused with
The same wazamono list records separately 信濃大掾忠国 (Shinano Daijō Tadakuni), who signed 平安城住忠国 (Heianjō jū Tadakuni): he is another swordsmith, whose tachi with mountings are at the 東照宮 (Tōshōgū) of Tottori, according to Japanese Wikipedia.