I have received the following responses, which are very similar except for the city name Fukuoka vs. Yanagawa and the word junin vs. ju.
Yanagawa ju Hisakuni saku
Probably this wartime smith:
Hisakuni (久国), Shōwa (昭和, 1926-1989), Fukuoka – „Hisakuni“ (久国), family name „Koga“ (古賀), born 1895, rikugun-jumei-tōshō
Yanagawa junin Hisakuni saku