It could just be the camera back worn down from use. There's an old Kodak studio camera in our collection like this...it's pretty obvious, there are grooves under the spring back from holders being slid in & out from years of use. I've never seen anything like it--the camera was in use in a portrait studio for almost 50 yrs. They shot thousands of negatives on it and literlly wore it out. It wouldn't take much for a holder not to sit right though...