According to Robert J. Neal in his article, The Mexican - An S&W Mystery, he states: Early guns used a trigger width which was .197” wide and a guard of proper design to match it. As the width of the hammer was .225”, the rear portion of the trigger did not use the full width of the notch surfaces of the hammer. This could cause a trench to wear on the hammer notches and result in erratic sear operation. Later guns used triggers the same width as the hammer to eliminate that possibility.
Dave