Can you give us an exact measurement of the diameter? If it measures 7/8" then it would be an officers button and that size was produced from 1851 until 1902. If it is smaller than that it is most likely an enlisted man's button and was produced until 1854. In 1854 the letter designation was removed from all enlisted men's buttons. It could still have seen service in the civil war because the letter buttons were not taken out of service and existing supplies were used until they ran out.