NNP

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

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor