If I understand what you describe, as far as rust bluing is concerned, it is not a sole cause of the rust effect you see--it occurs with salt bluing as well. If you do a search on Simichrome (or Semichrome) Test or Maglight Test you will find a discussion concerning this phenomenon. In short, the rust which you see is characteristic of an old blue--perhaps the original finish, or perhaps an old reblue--this test will not indicate authenticity, only age, of the finish.
--Dwight
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