They were given Model numbers around 1957. When the first change was made, it was called a 17-1. That was changing the extractor rod from right hand to left hand threads. That was made in 1959 and continued only into 1961, when they deleted the trigger guard(fourth) screw and changed the cylinder stop(dash two). The dash two variations were made til around 1967, with the relocation of the rear sight screw. You should be wearing diamond magnas, or quite possibly diamond target grips. Quite a nice gun, though I liked the four screw and five screw(top of sideplate) variations for collecting. Hard for me not to shoot a dash two and up, which is a good thing at the range. You did good. There are always exceptions to the rules. You might find a four screw 17-2, as they used up the frames they had.