"If you're referring to a wear line around the outside of the cylinder between the indexing notches, it is an indication that the revolver is out of time and raising the stop latch too early, letting it drag on the cylinder outer surface ILO into the notch's leade.
On Smith & Wesson DA revolvers, that is a sign of normal operation of the action. It absolutely is NOT a timing defect. Anybody from Smith & Wesson will verify this including the smiths in their Performance Center. That said, there are ways to minimized but not eliminate that type of wear. On revolvers of different manufacture, the story is different.