Difference between an M28 and an M28-2
The dash number indicates engineering/production changes. When this model was first introduced, it was called the Highway Patrolman. In 1957, S&W decided that instead of naming each revolver, it would be better to give them model numbers.
Thus the HP became the M28. As time went on, improvements (or just plain manufacturing shortcuts) were incorporated. Dash numbers ( -1, -2, etc.) were introduced to designate what features a particular revolver may be reasonably expected to have. For example, you may note that your revolver has a pinned barrel and each chamber in the cylinder is recessed to accommodate the cartridge rim. These features were eliminated in the M28-3.
Of course, there was not always a clear, clean break from one dash number to the next and you may find some transition revolvers that have the new dash number but retain the old features and vice-versa.