I need help in identifying letters that are stamped onto my M 1860 Spencer Carbine. It has letters stamped on the barrel, trigger guard and receiver and I am trying to determine their significance, if any. The letters "R" and "S" are stamped on the left side (saddle ring side) of the barrel very close to the receiver. The letter "M" is stamped on the trigger guard and also on the receiver very close to where the receiver is screwed into the stock near the saddle ring. I don't know if they are some type of inspectors' marks, there are two visible inspector cartouches on the stock - MMJ for Martin M. Johnson and DAP for Dwight A. Perkins. I am hoping that someone with greater familiarity with the Spencers can shed some light on this. Thank you.