長船三作 Osafune Sansaku

What is Osafune Sansaku in Japanese Katana terminology?

"Osafune Sansaku" refers to the three representative swordsmiths who were active from the middle to the late Kamakura period among the group of swordsmiths in the Osafune region of Bizen Province (currently Okayama Prefecture). These include "Nagamitsu", "Kagemitsu", and "Sanenaga".