For the 1971-S cent shown in the above linked article, the cupped depression is smooth. Would this still be the case with a clad coin? I have found this 1984-P half dollar that got me wondering.
Brinell hardness test (used for metals) is similar. The ball diameter is standard; I don't know what that is, but it is in the same ball park as a Biro ball.
Which brings me to another point:- it could be an indent by a Biro pen, but I don't think so.
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