I can't remember seeing that before but it must be more common then I realizeIn the process of rebuilding my control arms I noticed this dimple in the driver's side lower arm. It's the size of a hammer head but I doubt that any human could swing a hammer hard enough to leave a mark this deep in metal. I don't have the car here to see if anything on the driver's side needs clearance but I don't remember anything there. The passenger side doesn't have the dimple at all.
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. Here's a random picture I just saw on Facebook