The theme is described as Kachimushi 勝虫 no Zugura 三光の図柄. This theme of the dragon fly or Tonbo was popular the military men of Japan for at least 700 years. To the samurai, kachimushi were thought of as creatures that never took a step backward, but rather always moved forward, and because of that, were a preferred design and they were often used as a motif on tosogu, maedate (前立 plume or decoration on a jingasa) and on Jinbaori (陣羽織 sleeveless jacket) as well. This kozuka also has a thin Kibata (際端 edge or side) to emphasize both the relief and the strength of the design.
The tsuba is an excellent early Muromachi period Ko Tosho 室町時代初期 古刀匠 sukashi tsuba. Heavily modualted and rich in tsuchime 槌目, this tsuba displays many of the traits of th eearly Kamakura period 鎌倉時代 guards in both Sasano and Kremmers books. It is possible that it is from the Kamakura priod. The mimi is Kaku mimi koniku 小肉角耳 and there is tekkotsu 鉄骨. There is a single hitsu-ana (片櫃仕立 – Kata-hitsu shitate) for a Kozuka ana. The conventional wisdom is that any kozuka ana must be a later addition if the tsuba is early. I suspect that many are original.
The kozuka is Ko Kinko and from the Muromachi period (古金工 無銘 小柄 室町時代). Kachimushi (勝虫 dragonfly) theme. Shakudo nanako-ji, takabori iroe (赤銅 魚子地, 高彫色絵). Featured in the book Jidai Mitokoro Shu published by the Nihon Tosogu Bijutsukan (日本刀装具美術館 Japanese Sword Fittings Museum). This piece is a little shorter than works of the Edo period, but its width is exaggerated in comparison with the length. This Kachimushi design is placed nicely in the center of the kozuka. This design trait is seen in a lot of works from the Muromachi period from both the Ko Kinko and Ko Goto groups. It is an excellent piece with a majestic figure and shining eyes.
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