19mm, North Carolina Button also known as the  "Nolime Tangere" North Carolina (...and to some, as the "No limes or tangerines" button!) this is one of the Big Five.  It is so rare that most Civil War collectors have not seen it and many uniform button collectors do not even know of its existence.  Ultra R-a-r-e!. 
Be sure to scroll across the face to see the reverse.