It was in my original carbine, traced by Two Flints: An exact match for your Spencer Carbine, according to my SRS BOOK. Your Spencer Carbine serial # 30337 was issued to a member of Company G, 6th Indiana Volunteer Cavalry on July 13, 1864.
But since it is a centerfire upper block, it came later. I was wondering if anyone recognized the make and if it is suppose to have a spring mechanism of some sort to return the firing pin. It is just loose now, and slides back and forth freely.