Ko-Bungo (古豊後) names the early phase of swordmaking in Bungo Province on Kyushu, rooted in the foothills of Mount Hiko (Hikosan) and active from the close of the period into the early . The place its leading figure, Yukihira (行平), within the Kyushu classical group (Kyūshū koten-), recording him as a (assigned smith) serving the Retired Emperor Go-Toba, and preserving his styling as Kishin Dayū or Shintayū. His link to the Hikosan monk Sadahide (定秀) is transmitted in both directions, some accounts naming Sadahide his teacher, others his pupil; further notices give his master as the Hikosan monks Teishū (定秀) and Jōshū. Sadahide himself appears in the register under signatures reading "Bungo no Sō Sadahide," a monk of Mount Hiko whose forging is described as closely matching Yukihira's. The line continues into a and attributed to Chikushi Masatsune (筑紫正恒), recorded as Yukihira's grandson. A dated Genkyū 2 (1205) anchors Yukihira's working span, while the entries align the group with the most archaic temper, citing the Shōsōin treasures as the model these Kyushu hands transmitted.
The collective manner is consistent across the , , and of the register. is slender () with a , high , and clear at the base, the curvature settling shallower toward the point in an archaic shape held to not descend beyond early . The is mixed with and at times , dense and soft in character, with a (viscous, clinging) quality; gathers thickly, enter finely, and a whitish tendency stands in the ground. The is -based, often carrying a shallow and mixing in , , or ; enter, adheres, and faint , , , , and appear. The defining trait of the edge is an (moist, blurred) , paired with a in which the temper drops above the , sometimes a large one. is a further marker: relief carving () of , and at times Jizō Bosatsu, , the "pine-eating crane" (matsukui-zuru), and cherry blossoms, set within or at the . The entries note that such motifs are not seen on works earlier than Yukihira.
For the register is explicit. Yukihira and Sadahide habitually cut the on the (the side worn outward when mounted as a ), against the common practice of their day, a placement the list alongside Munechika and the descendants. The combination of soft steel, edge, and is repeatedly named as the shared ground that also resonates with other old Kyushu work, Ko-Hōki and Ko-Naminohira being cited as . The entries caution that forgeries circulated from times and that Yukihira's genuine signatures are plainly cut, so finely written invite doubt. Provenance runs through the imperial collection, with of Yukihira held by the Imperial Household Agency, a Sadahide ko- carrying scattered -tomoe crests on its , and a Sadahide transmitted as a gift from Emperor Go-Mizunoo to the Mōri house; one Sadahide blade bears an owner inscription beside the maker's signature, and a Yukihira descends with an -crest mounting. Several pieces are published in the Kōzan . The state that this carving method and passed on to the later Bungo (Takada) hands, marking Ko-Bungo as the headwater of the province's tradition while keeping its early work distinct in its archaic grace.