The Mogusa school (Mōkusa, Maikusa, 舞草) belonged to Ōshū, the old province of Mutsu in the far north of Honshū, and is among the most archaic of Japanese sword lineages. The trace the name in documentary sources to the period, and in the case of the signed Mōkusa the editor notes that the name is attested as far back as the Nara period, though extant signed works are very nearly without parallel among survivals. The blades preserved here are read as Ōshū work that does not descend later than the period: a two-character signed read Mōkusa, an transmitted as Maikusa, and a signed Toshiyasu (世安), who is counted among the Maitsukusa smiths whose beginnings the place in the period. The Toshiyasu blade is itself judged a late-, later-generation example within works bearing that signature.
Across these three blades a consistent northern vocabulary emerges. The is the school's signature: a flowing in which the grain stands up conspicuously (-tatsu), the figure often large and raised, with a -inclined tendency running through it; and gather in the steel. The temper is restrained, a to ground that admits small () and a shallow undulating, almost -like movement, carried in small with the tending toward tightness or toward a subdued character. The runs to or with at the point, and on one takes a -like turn on the . To recognise Mogusa work, look for this raised, flowing standing grain over dark steel and a quiet temper of mixed with small irregularity, an effect Honma described in oral commentary as old-fashioned and rustic, and which can at first glance read as without being so.
The value of these pieces lies precisely in that archaic, out-of-place impression: the observe that careful step-by-step appraisal of and leads to an Ōshū origin, and that the few genuine signed Mogusa blades resemble one another in just this way. The smiths named in the corpus are Mōkusa and Toshiyasu of the Maitsukusa group, with the read Maikusa. Two of the carry northern provenance, one held in Akita by Kudō Shigemi and the Toshiyasu blade once held at Osaka by Kashiwabara Jinbē and recorded in Nihon no Meitō, Nihontō Taikan, and no Mikata. The rate the documentary value of these blades as exceptionally high, since signed Mogusa work is among the rarest of the early Ōshū lineages. Within that northern sphere the school stands beside and the Gassan line as a founding current of Mutsu sword forging, its surviving blades read as direct evidence of how early steel was worked in the far north.