Yours is from about the time when the so-called PC (plain clothes) grips started to come into use, and I would strongly suspect yours has a square butt - round butt M&Ps were a little uncommon at that time. Anyway, PC grips are sort of rounded on the bottom, and do not have sharp corners. The idea was that concealed handguns would not print through clothing as much and also would reduce wear on jacket linings.