Springfield Armory M1873 Trapdoor Carbine

Recently picked this up at the Greensboro show. Looking in the values bible the rifle fits most of the requirements for being a Custer era piece (ser # below 40,000. ser # is 147xx). OK, all well and good, but there's a slight glitch. The Last Stand was dated 25 June 1876 and one of the cartouches on the rifle is clearly dated 1881, 5 years later. What does this mean? Is this a Custer era piece that was refurbished? Any help and value estimate will be greatly appreciated.