An orthotropic lattice structure interconnects porous surfaces of the flap with internal lattice-structured perforations to equalize the steady pressure field on the flap surfaces adjacent to the end and to reduce the amplitude of the fluctuations in the flow field near the flap end. , including the step of reacting a compound represented by the formula :wherein X represents a chlorine atom etc. This recess serves to suppress the vibration mode of a disk tray and the air flow between the disk and the disk tray.