If chamber length is not marked on the gun, then I'd date it 1904-25 rather than 25-54. After 1925, the chamber length would have been marked. (Good way to differentiate between 1904-25 and 1925-54 guns.) The 12 over C mark does not mean 2 1/2" chambers. Rather, it means chambers shorter than 3". Guns with 3" chambers were marked 12 over LC.