Glad you got your gun back, in proper working order. "Replaced cylinder" is what they did, which I believe they are required to tell you. We all know why they had to replace the cylinder, but the how on how it got that way in the first place we'll probably never know.
S&W would likely consider that proprietary info, and as such won't be sharing it with the world at large. probably will be sharing that info vigorously inside the company, especially in those depts involved in making your gun, but we won't hear about it, unless you have an "informed source" inside S&W....
Gun is fixed, problem over, right?