The caliber is 56-50. The top of the breech block is indeed wider to accommodate the Spencer cut-off. I looked up the serial number on SRS, but most of the 4 digit SN's are rifles. Is it possible that this started out life as a rifle, or did Spencer actually produce M1865's post war? I was under the impression that all Spencer M1865 carbines were refurbished M1860 rifles and carbines. Thank you all!