What does model 637-2 mean in S W 38?

In Smith and Wesson code, the 6 means the gun is primarily made of rust resistant stainless steel. The 37 designates the lightweight version of the model 36 .38 caliber revolver, and lightweight guns have aluminum frames. The -2 designation means that this revolver is the second version of this model. So 637-2 means a stainless barrel and cylinder on an aluminum frame based on a lightweight model 36 revolver in the second version (or first design revision) of this model which is designated model 637. The model 36 and model 37 Smith and Wesson revolver are 5 shot revolvers with a 2, 3, or (recently) a 5 inch target barrel. The 2 inch barrel is the most common version. The model 637 Smith and Wesson 5 shot .38 caliber revolver is only available in a 2 inch barrel.