The Tadatsuna
According to the Japanese Wikipedia, the first Tadatsuna moved to Osaka, received the title of Ōmi no Kami and claimed descent from Awataguchi Kunitsuna; he and his son signed 粟田口近江守忠綱 (Awataguchi Ōmi no Kami Tadatsuna). The second, Ikkanshi Tadatsuna, worked in the Genroku era (1688–1704). The same encyclopaedia counts the school of Tadatsuna among the smiths of Ōsaka shintō, known for the beauty of its steel.
The first
The Japanese Wikipedia does not give his dates of birth or death, and says that he is held to be the younger brother of Ōmi no Kami Tadatsuna. The same article records, without deciding, two versions: that he was the brother of the first Tadatsuna or of Ikkanshi Tadatsuna, the second; and in another passage it speaks of “his father Tadatsuna”.
According to a theory recorded by the same encyclopaedia, he made blades for Tadatsuna with Tadatsuna’s signature; that would be why there are few long swords with his own signature, and those there are closely resemble Tadatsuna’s in signature and in workmanship.
The generations
According to the Japanese Wikipedia, the name was kept for several generations. The first signed with the surname Fujiwara, 摂津守藤原忠行 (Settsu no Kami Fujiwara Tadayuki), and before the title, 摂州住藤原忠行 (Sesshū jū Fujiwara Tadayuki); from the second on, with the surname Minamoto, 摂津守源忠行 (Settsu no Kami Minamoto Tadayuki). The sources consulted do not give how many generations there were, nor their names, nor their dates.
Who he is confused with
He is not the first Tadayuki of Hizen, son of Ōmi Daijō Tadahiro and smith of the Ogi domain, according to the Japanese Wikipedia. Tomita Kiyama, in his ranking of shintō smiths (1933, published in 1935), places among the maegashira of the west a “Sesshū jū Tadayuki” of Osaka; it is the signature without the title, and it cannot be said for certain that he belongs to this lineage.
In the lists
The Kaihō Kenjaku (懐宝剣尺) of 1797, the Yamada cutting list, places the first Settsu no Kami Tadayuki among the 50 yoki-wazamono (良業物, yoki-wazamono), the third of its four grades, according to the wazamono lists of the Japanese and English Wikipedia. He is not recorded in the Kyōhō meibutsuchō (享保名物帳, Kyōhō meibutsuchō), nor is any work of his recorded as designated a National Treasure or an Important Cultural Property.