Having a K (or I or J or N) in the serial number was true for a while, but as with so many "facts" about S&W it is only partially correct. There are guns that were built before they gave them the model numbers (10, 29 etc), and guns that have frame size in the serial numbers, and guns that do. It is sort of daunting at first, but eventually you will get it all if you just keep reading. A history of S&W by Jenkins and the standard catalog of S&W are good places to start. Roughly, from smallest to largest, (and avoiding the volcanic and 19th century single and double actions, though even here there are exceptions) they go I, J, K, L, N, X. I frames haven't been made since before WW2, but the rest are all still around. Hope this helps a little. - dan