There is no Trooper Mk II. The original Trooper was made in .22 LR and .38 Special, with the Colt .357 Magnum being the same revolver in .357 Magnum. When the Python was intoduced, desire for the .357 Magnum declined, and the Trooper was then made available in .357 Magnum, along with the previous calibers. I can see how the revised Trooper could be thought of as a "MK II". Anyway, the serial number corresponds to a 1964 Trooper; .357 Magnum revolver production ceased in 1961. Trooper Mark III production started in 1969.