Each time S&W made an engineering change that affected interchangability of parts, a new dash number was issued. The 686 started off with the basic 686 model number then went to 686-1, 688-2 etc.
Each engineering change on the 686 was done to reduce manufacturing costs. As such, the lower the dash number, the more desireable the gun. The history on the 686 is a bit different than other models. The no-dash and early dash 1s had a recall for the firing pin bushing. If you find one of these, make sure it has a "M" stamped in the frame by the model number to indicate the factory modification was completed. The dash 2s had a funky floating hand that was restored to the standard hand with the dash 3s. The dash 4s came with elcheapo Hogue rubber grips instead of wood target grips and a goofy extractor. After the dash 4, S&W started using MIM parts (hammer, trigger, and a few more) to cheapen them even more.
The best 686s were the no dash through the dash 4.