I'll take a stab....If you mount the brass spacer with the flat side against your screw it will eventually wear into the wood and your servo will vibrate a hole right into the wood. With the brass 'washer' on the bottom this is prevented and the tightness of the screw will never change as you're actually tightening the screw aganist the washer and the plane instead of just pushing it into the wood. You are in essence suspending the grommet and thus the servo between the screw and the brass spacer rather than squishing it against the plane...