It has to be the pressure plate (back plate).I had a similar problem with an old Argus Brick. An online source as well as my local camera shop suggested using Emery grade sand paper. Its for light rust removal.
Thats my 2 cents worth but I am sure you will get better advice. One thing you must remember.......after the metal is sanded it will be prone to rust if you expose the camera to a lot of moisture. The sanding removes the Blueing from the metal.

Good Luck,