Checkman;
There is a problem with some Smith & Wessons'
They would serial a certain size of frame (such as a J-frame) and them use the frames to make different model guns. Using a reference book, such as Jim Supica and Richard Nahas's excellent "Standard Catalog of Smith & Wesson" one can determine that a certain serial number is within a range of numbers that were made between certain years.
For example your model 49, #J 1517xx fits within a range of numbers running from J 100000 to J 250000 made during 1973 to '74 - as Mike pointed out. But besides the model 49 all other J-frame revolvers, such as the models 36, 37, 38 and 50 were included within the same serial number series.
The only way to tell exactly when your gun (or any other S&W) was made is to pay a reasonable fee of $30.00 and obtain a detailed letter from Smith & Wesson's historian, Roy Jinks.
Do buy the book. You'll learn a lot more about Smith & Wesson's then just serial number information.