I had this exact issue a while back. It was in my Yashica Electro 35 GSN and Kodak Gold 200. I take my film to a professional lab and they were stumped as well. I gave them the camera and told them to hang onto thenegs to find out the issue, they tracked it down to poor handeling during development. Camera was ruled out because the scratches weren't all in the same place or direction like the pressure plate would cause.