This signed katana reads 筑前国福岡住是次 “Chikuzen no Kuni Fukuoka-jū Koretsugu,” with a blade length of 60.6 cm and a curvature of 1.45 cm. 
Koretsugu (是次) was a distinguished swordsmith of the Edo period and a retained smith of the Kuroda clan of Fukuoka. Together with Moritsugu, he is regarded as one of the leading masters of the Fukuoka Ishidō school. A descendant of the Bizen Ichimonji lineage, he was the son of Toshihide and born in 1609. Known also as Hansaburō or Ippei, he studied in Edo under Sakon Koreichi before returning to Chikuzen to serve the Kuroda family. He passed away in 1681 at the age of 53. His work is characterized by a fine ko-itame hada.

The blade features a suguha-based hamon with gentle undulations and clearly visible nie, giving a bright and lively appearance. The jihada is a refined ko-itame, consistent with the smith’s typical workmanship, and the overall sugata reflects a balanced and elegant Edo-period style.
 
The sword is mounted in Type 98 guntō koshirae with an iron scabbard. The combination of an older traditionally forged blade in wartime mounts reflects a continued functional history across periods.