That "mark" on the cylinder isn't a "mark", it's a complete and total FAILURE on the part of Q.C. at S&W. Contact S&W by phone because they tend to ignore email. They will then issue an RMA and instructions for returning your revolver to S&W one THEIR dime, meaning you don't pay for a warranty return. Once at S&W they will replace that defective cylinder at no cost to you. Turnaround is rumored to currently be about 6 weeks but is actually typically closer to about 2 weeks. Meaning, you'll get your repaired revolver back in 2 to 6 weeks.
As for the sighting issue, these revolvers do NOT come from the factory with the sights adjusted. You didn't mention that you were trying to sight in your revolver, just that you couldn't adjust the rear sight any lower. I would suggest that you actually take the time to sight in your revolver before sending it in. I suspect that if you take the time to sight in your revolver you may find you need to raise the rear sight to get it on zero. If you have attempted to sight in your revolver and found the rear sight blade is too tall and you need a shorter rear sight blade installed that way it can be done at the same time your cylinder is replaced.