Like many early cards that have been soaked(primarily due to scrapbook removal), many high-grade and of course lower grade coins have been cleaned in their past life. Some aggressively with silver polish, brasso and other cleaners, others "whizzed" with wire brushes and many, many "dipped" in jewelry cleaners such as Jewel Luster and modern days products such as MS70 to remove toning or other tarnish.
ALL will affect the grade by a 3rd party, many will come back with "cleaned" or improperly cleaned label and bring a fraction of the price of a true mint state or high-grade coin.
Like cards, there are a myriad of other coin doctoring methods from simple retooling of designs to using lasers.
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