The "dash number" is a way of designating guns through years of production as having certain modifications that were carried forward to all in that model thereafter. It works similarly to suffix letter designations on aircraft.
For the model 15:
15 no dash........ 1957 stamping of model number
15-1............... 1959 left handed extractor rod threads
15-2............... 1962 delete trigger guard screw change cylinder stop
15-3............... 1967 relocation of rear sight leaf screw
and so on.......
Some changes such as the diamond disappearing in the grips did not merit a model number change, as the parts to repair the gun were still the same. It's really not an issue of collector minutia, but the need to specify which gun so proper parts can be ordered.
FWIW, prior to the Model number era, the changes were listed as changes, for example.....Model of 1905 third change. or M1905-3rd Change
Oh........