Ko- (古三原) opens the story at its Bingo base, where a body of smiths gathered from the close of the period and through the era. The treat the term as a chronological bracket rather than a separate workshop: works of this span are "collectively termed Ko-," with Masaie and Masahiro repeatedly named as the leading figures, and Masahiro held by tradition to be Masaie's son. The province's web of shōen estates tied to Kinai temples such as Tōji and Rengeō-in (Sanjūsangendō) drew the early smiths into regular contact with the central provinces, and the temperament that the records read in these blades is referred to that exchange. A second current runs alongside it: because some pieces show an manner, the appraisals also weigh influence from neighbouring , as the Chikatsugu of Shōhei 7 (1352) makes plain, sitting on the seam between Yamato bearing and surface.
In the , the early hand keeps an -tending ground that takes on , , and a -like flow, with the grain inclined to stand () and fine set densely across it; a pale recurs as the most cited diagnostic, though the Masaie of around Jōji instead shows a darkish steel with a -like cast. The temper holds to a refined with and , the drawn tight or , the breaking into , , uchi-noke, and a suggestion, with fine and threading through. The runs , closing in , , or with at the tip. The records keep these early works the cleaner pole of the school: a quiet, controlled Yamato manner whose forging shows "not the slightest looseness," set against the coarser, looser later Sue- of the closing , where the disciplined and tight ground give way.
For , the early phase reads as Yamato bearing without Yamato vigour: the of and runs weaker than in the Nara schools proper, the steel turns whitish, and the tightens, so a bright clean with pointed can be mistaken at a glance for until the broad , high , standing , and with declare . Masaie and Masahiro divide on : Masaie is the man of the bold and the , while Masahiro keeps ordinary proportions and a freer, edge with deeper , as in the Ō- cited by name. Because Yamato habit left few signed works, the early phase rests heavily on and attributions, several of them given to Masaie by Kōtoku, while dated and signed survivors stay scarce and so carry weight as reference material. Provenance threads through the great houses, the signed Masaie presented to Emperor Meiji by the Shimazu family and another Masaie transmitted in the Date.