The Smith & Wesson firm was the American "pioneer" of double-action semi-automatic pistols.  The 9mm Model 39 was the very first mass-production pistol of that design, introduced in 1954.  It quickly earned a reputation for being a very well-made pistol that was both reliable and accurate, with many Law Enforcement agencies all over the U.S. adopting it for their service weapon.  The pistol remained in production as the Model 39 until 1970, when S&W made some changes to the model and added the "dash 2" to the name.  The changes included redesigning the extractor, barrel guide rod hole, and feed ramp to improve the functionality of the gun.  The Model 39-2 stayed in production for another 12 years until being discontinued in 1982