The Dash marks on S&W frames denotes changes. The 27-1 was removing the the top screw in the side. The 27-2 was changing the thread pitch on the extractor rod. And on it goes. The problem with aging is frames were often used long after changes were completed. You will also note that serial numbers change from "S" to "N". I have a Mdl. 29 no dashs but has all the -2 changes with an"S" serial. The S&W serial number will give a more accurate age.