Hi Atlantia,
Thank You for Your thoughts.
I try to give some answers. I received the kudi polished, with the trucks of file You saw, so I don't know the oxidation degree it had.
I looked under some patterns, so I etched the blade before with citric acid and then (only few seconds with a brush) with ferric chloride diluted 1 : 3 with distilled water (the spine has brass and copper inlays). I don't think that the patterns are dued to the rust (or pitting) removed (but it's only my opinion).
I don't want to remove the blade from the shaft; You are right when say that "... patterns diminish where the blade looks to have been filed "
So
Paolo