It's a Model 10, the most produced gun of any Smith and Wessons and has been made continuously since about 1908. Pobably used by more law enforcement agencys than any other gun in it's time.

Since you couldn't tell what it was, I presume it isn't stamped Mod 10 on the frame in the yoke which means it pre dates 1957. And since it's a 4 screw it post dates 1955, so that will give you an idea of when it was made.

__________________
Jim
S&WCA #819