While I don't take issue with what's been posted so far, I would like to point out that dash numbers have sometimes been used to designate options, generally barrel lengths and types.
For instance, the Model 36 no-dash had a 1-7/8" or 3" tapered barrel, whereas the Model 36-1 was the same gun with a 3" heavy barrel, and was sold simultaneously (more about the Model 36 below). Another well-known example is the Model 10-5, which had a 2", 4", 5", or 6" tapered barrel, and the M10-6, the same gun but with a 4" heavy barrel.
It should also be noted that S&W was not always consistent about changing dash numbers when engineering changes were made. Arguably the most well-known example is the Model 36, which was produced from 1957 until the mid-1980s in no-dash / dash-1 form despite the fact that numerous changes were made during that time, most notably the deletion of the barrel pin in 1982.
"Smokey, this is not 'Nam. This is bowling. There are rules... MARK IT ZERO!!" - Walter Sobchak