It is a Smith & Wesson Safety Hammerless (aka "lemon squeezer") Fifth Model, made between 1907 and 1940, probably around 1920, but it is impossible to get any closer on the date without having S&W do research. The serial range is 220,000 to 261,493, end of production for the .38 Safety Hammerless series. With some 41,500 Fifth Models, they are fairly common, and, in average condition, bring only $100-150 or so.
But in top condition, as yours seems to be, they will bring $500 or even more at retail.
The cartridge is .38 S&W, not .38 Special. Ammunition is available, but I advise not firing the gun since anything that breaks is nearly irreplaceable.