The first modification was the replacement of the of the firing pin bushing, and resulted in both of my CS-1s being stamped "M". From what I recall, the busing was replaced due to primer material occasionally flowing into the bushing/around the firing pin. The work was done at S&W; both guns shipped through my LGS who was a factory-certified S&W armorer.
I sold both guns either before the second recall/mod happened or I was not made aware of it.
The guns were very sensitive to powder under the extractor star. They did have excellent sights; some of the best that I recall having on a Smith revolver.