S&W started model numbers in 1958 although a few model numbers started to appear in late 1957.I should know this , but when did the (pre 10) M&P become the model 10?
One book that I truly benefit from owning is the Smith and Wesson Standard Catalog by Supica and Nahas. It will give you much more information than when your gun was made...but it will do that, too. If you like old guns, you owe it to yourself to buy it or ask Santa for it.