I have a Masamune and this looks like the Nodachi to me. Hope you can do something about that staining. Mine has never gotten like that, and I don’t remove the blade and clean each piece every time.
Yours is a Nodachi top cap. Here's a picture of my Masamune top cap. The Masamune cap has longer blade tabs and is not threaded all the way. Someday I'll get a Nodachi top cap for my Masamune base plate and can make a side-by-side picture.
Thank you all. I don't have another cap to compare it with. The threading doesn't go all the way to the cap body, but it seems the fact that the tabs are shorter indicates it is definitely the Nodachi. It was purchased second hand, and I've always felt it to be a fairly mild razor compared (for example) to my Blackbird or Karve F plate, hence I started to wonder. Cheers!