I cut em off with a bandsaw, then put them in a lathe nd true them up and profile. But, you could use a file and a ball. Before you go to valve grinding compound work your way through finer grits of emery cloth wrapped on the ball'

Round butt. First check that serial number will survive in tact. Then install round butt grips and scribe a line on both sides of the frame. If you don't have a good scribe any good hard steel ground to a point will work. The tip of a file handle ground to a sharp point, heated bright red and then dunked in water will be very hard. Grind close and finish with finer and finer paper.