20mm, General Service Hard Rubber, Berdan Regiment of Sharpshooter Button with a "N. R. Co./Goodyear's P-T. 1851" backmark.  The idea of using the subdued color of gutta percha was to counter the reflective quality of gilded buttons.  This was a Civil War era nod toward camouflage (Berdan's coats were dyed green, as well!).  The "older collectors' had shared with me that the lye soap available to soldiers turned them brown.  Interestingly, this one is brown to the front and black on the back... maybe it was UV light interaction that changed them?  If you have a chemical background and know, please let me know.   In either case, this is indication that this one went to the field and made it back.