PART  IV. Die Errors:

Rockwell Test Mark Left in the Die

See Planchet Errors HERE

Definition: A common device used to measure the hardness of metals in the U.S. Mint is the Rockwell hardness tester. It measures the depth of penetration of a steel or diamond-tipped pin relative to the applied force.

A Rockwell test mark will appear on a die face as a small pit.  On a coin it will appear as a tiny dome-shaped or cone-shaped elevation.  It is most likely to appear in the field.  No unambiguous examples of Rockwell test-marked dies are known among U.S. coins.  This is not surprising as a Rockwell test mark is very hard to distinguish from a small circular die dent.