Private Mark. A letter or symbol hidden in the design or signature of a coin or medal which has some private meaning to the original artist; or applied to an existing numismatic item by a onetime owner. In the earliest recorded case of this, the French medalist Louis Merley added an "M" to his signature, thusly: L. Merley.F.M. Some owners have added letters to items in their collection (their initials as a collector’s mark say), but the practice is not encouraged and may be considered a form of graffiti to subsequent owners.
excerpted with permission from
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor