I have no idea what it says. I would like to know.
Probably move this to scraps later.
EDIT: A translation has been found.
備州長船勝光 (bishū osafune katsumitsu)
Bishū is an area in Japan, modern Okayama prefecture and Hiroshima prefecture. Osafune Katsumitsu is the name of the swordsmith, active from 1441-1444.
Considering the hardware with the rest of the sword, I'm guessing that it is a clever fake.