NNP

Off Metal.  A struck piece on a planchet of incorrect or unintended composition. It is sometimes called off strike; but more precisely called off metal strike, abbreviated OMS. But it has also been called off metal composition, abbreviated OMC.

Reference:                                                                                                                                      

NE42 {1982} Doty, p 238.

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