There are some theories about 'swords made with a forging method of Koto (Old Swords) proposed by Suishinshi Masahide,' but generally this indicates swords in the late Edo era after the late 18th or the early 19th centuries.
「水心子正秀の提唱により制作された古刀の鍛錬法を用いた刀」など諸説あるが概ね18世紀末か19世紀初め以降の幕末頃の日本刀をさす。