The -making traditions of Choshu domain (Nagato Province) trace their origins to the migration of Heianjo- openwork craftsmen from Kyoto, invited by the Ouchi family of Suo and Nagato during the period. This transplanted lineage gave rise to the Ko- group, which flourished from the late into the early period and established a distinctive regional idiom in iron openwork. Under the subsequent patronage of the Mori clan, Choshu developed into one of Japan's foremost centres of production, earning the epithet that placed it alongside Aizu as the two great guard-making provinces: notable both for the number of artisans and the volume of work. Multiple lineages emerged — among them the Kawaji, Nakahara, Hatto, Okada, Okamoto, Kaneko, and Nakai — with Nakai Tomotsune (also known as Zensuke) achieving particular renown as a kakae-ko (retainer craftsman) of the Mori household, receiving a stipend from Horeki 8 (1758).
The collective vocabulary of the Choshu tradition is grounded in iron: thin, carefully forged plates with rich surface character, including (hammered ground). The Ko- group worked almost invariably in combined positive-and-negative openwork (in' ), rendering branching chrysanthemum () compositions in which the fine linear passages of the negative openwork recall Kyo-, while the positive portions are treated in a distinctive shallow relief (usu-) that produces what the describes as "a well-balanced union of delicacy and lush charm, together with a subdued taste and softly refined elegance." The later Nakai school extended this foundation into a broader technical range, deploying high relief, (gold inlay with polychrome accents), , and gold in compositions of considerable ambition — naturalistic subjects such as frolicking tenaga-zaru monkeys and theatrical Shiki Samban scenes rendered with "taut, controlled modeling" and effective distribution of colour metals.
The Choshu tradition occupies a singular position within the tosogu canon as a regional milieu whose finest productions transcend mere provincial category. The has recognised that the best works by Nakai Tomotsune surpass "what is ordinarily encompassed by the category of so-called ," distinguished in composition, carving technique, and the character of the iron () alike. At the time, the Ko- group preserves an older and quieter aesthetic — rooted in the Kyoto openwork heritage yet transformed into something "not seen in other makers' work" — that testifies to the depth and continuity of metalworking practice in Nagato Province across several centuries.