The Gassan family
The modern branch of the Gassan school was refounded around 1830 by Gassan Sadayoshi (月山貞吉, Gassan Sadayoshi), who settled in Osaka in 1833. His son, Gassan Sadakazu, was an artist of the imperial household (帝室技芸員, teishitsu gigeiin), and Sadakatsu, born in Osaka in 1869, was the latter’s eldest son. Sadakazu died in 1918 according to the Japanese Wikipedia, Kotobank and the German Wikipedia; another database of the Japanese state labels his dates as 1836–1912.
Training
He learned the craft from his father from a very young age. Until 1918 he did not sign with his own name: he made work to order under his father’s name (代作, daisaku), and the German Wikipedia adds that many blades with Sadakazu’s signature may in fact have been his (代銘, daimei). The oldest signed work known by him is a wakizashi of 1918 kept at the Dewa Sanzan (出羽三山, Dewa Sanzan) shrine; that same year, on his father’s death, he took over the Osaka house.
The court and the Ise shrine
From 1921 the imperial household commissioned work from him for the emperor, for the Imperial Household Ministry and for high-ranking military officers. In 1927 he and his son, who then signed 貞光 (Sadamitsu), received from the office of the Ise shrine the commission to forge, within three years, 68 tachi and 43 hoko (鉾, hoko) for the 58th ritual transfer of the shrine (式年遷宮, shikinen sengū); in 1929 they forged together the 大元帥刀 (daigensui-tō), the sword of the Emperor Shōwa as generalissimo. Markus Sesko tells this in his biography of the son. He also forged the protective sword (守り刀, mamori-gatana) of Crown Prince Akihito, according to the Japanese Wikipedia. The German Wikipedia says that he forged for other dignitaries, among them King George V of the United Kingdom.
Yoshino and the war
In 1934 the city of Osaka honoured him for his skill, and in 1935 he set up a new forge on Mount Yoshino (吉野山, Yoshino-yama), in Nara; the works with the signature 月山精練 (Gassan seiren) date from that time. He was an Army-designated swordsmith (陸軍受命刀工, rikugun jumei tōkō). His guntō were somewhat shorter, with a straight or undulating hamon, and in the Second World War he and many of his pupils supplied blades to the Army arsenal in Osaka.
Pupils
Trained under him were his son, who later signed Gassan Sadakazu like his grandfather and became a Living National Treasure in 1971, and Takahashi Sadatsugu (高橋貞次, Takahashi Sadatsugu), Living National Treasure in 1955. The Japanese Wikipedia gives Sadakatsu as Takahashi’s master.