Mopper is just another name for a polisher.
From "A Dictionary of Occupational Terms"
A Mopper, aka Grease Mopper was a polisher who used a mop of calico or other fabric, dressed with abrasive or polishing composition or grease.
A Bobber, aka buffer, buffing machinist, removed surface irregularities and polished metal articles by holding them against a power-driven revolving wheel, the edge of which was covered with cloth or leather and sprinkled with abrasive powder or polishing compound.
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