I have a couple of friends that have LCR's and they both became aware of this anomaly from a post on another forum. One of the two then asked my opinion. I told them that it was from machining the back of the barrel after pressing it into the frame. To put their minds at ease, I took them around to 4 different gunshops, and lo and behold, we looked at a total 9 different LCR's, including two of which were the newer model with the Big Dot front sight and the Hogue boot grips, and one of the models with the Crimson Trace laser grips. The end result? all nine guns had the mark in varying degrees.
Why would Ruger replace the gun? Your guess would be the same as mine, mere conjecture, but I think the fact their cost is minimal in view of the manufacturing processes to make that particular model, and the low cost to them would be a factor, for a little customer satisfaction. Just like McDonalds, somtimes you have to throw away some product to keep your customers from complaining.
I had to come back and edit in, that after playing with all these damn LCR's, I now have to have one of the ones with the Big Dot sight, and the boot grip. That is one sweet little gun.