Welcome to the forum. Can you post pics of this revolver?
I answered your post on the other thread before I saw this one. Your gun is a .38 Military & Police, third change. It was probably shipped around 1911-1912.
The M&P series is S&W's most successful line, with millions made over the course of a century. The descendants of this model are still being made today, though there have been evolutionary engineering changes.