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Damn! I like enigmatic mysteries!!!!

This phenomena begins with a circular ring of rust at the surface of a steel die formed due to corrosion at the edge of a tiny circular water droplet on the die. As this progresses it results in a circular ‘dot’ of rust on the surface of the die, which is pulverized and lost as the die is used in the coining press leaving a round pit. This shows up on the coin as a circular raised dot. The process has been documented in an article by Paul M. Holland in the Journal of the Numismatic Association of Australia, volume 27, 2016, pages 1-6.