If you inspect the negatives, does a scratch show? If you line up the negatives on top of eachother, does it show in the same place? I downloaded the images and zoomed in closer, and it looks like it could certainly be a scratch.
Do you have B&W processing abilities at home? Id suggest running a roll of B&W film through and processing yourself. Take the lab processing and their scanning out of the equation. If you still can detect the scratches on your home processed B&W film, you can be sure its the back.