Half shaft means get rid of half the center shaft. There are two sides to it and the plate sits in between them. You can cut one side off completely so there is only one side to it. You can swap to the countersunk screws after shaving out part of the screw holes in the shaft. I'm assuming knife-edge the plate means to slightly sharpen the edges of it so it's smoother. Stock it comes with a slight taper on the edge, I guess knife-edging exacerbates that taper to more of a sharp edge. Common sense.