I have a Mod 66-2 4" and the serial number on the bottom of the grip and the one behind the yoke are one number off. The number on the bottom of the grip is 308Kxxx and the number under the yoke is 307Kxxx. The last 3 numbers are the same in both locations. Is this normal? I bought this pistol brand new from a gun shop in the early 80's and it has never been to a gunsmith or had any alterations.