I've noticed that this happens to kettles that sit out, uncovered, with a lot of ash left inside. My theory is that the ash gets under the straps & against the spot welds. Ash holds any moisture it comes in contact with. Starts rusting the welds & then back of straps. They pull away from bowl over time, causing the dimples.
Edit: The straps are spot welded to bowl before porcelain coating, so I imagine that some of that metal is bare underneath.
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