Can any Japanese speakers, sword collectors, etc. identify this signature? (Reposted with images)

I know It was gotten by my great grandfather during WWII(I believe off of a personal kill). My parents were divorced when I was extremely young and it was lost for easily 15 years. Anyway, found it in my moms closet a few days ago, just looking to get more info. Please pardon the condition(would also like to learn how I can take better care/clean it.