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You were right, leather cant scratch steel like that. Flat out not possible, leather isn't hard enough. Stuff embedded in the leather, however, is more than capable of it. You might have some bits of grinding award, abrasive from sandpaper, something stuck in the leather, and as far as how it got there, there's a long list of suspects. Working with the leather on a dirty surface, storing the leather in range of your grinder, etc.
Best practice to avoid this in the future is make sure your leather and the work surfaces are all clean, to prevent it picking up something you'd rather it didnt
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