A hybrid system for identifying a printed page. The base plate attaches to the cover such that the base plate is secured on each edge of the base plate and is prevented from detaching. The rotor is mounted adjacent the outlet openings 11 and, at any one rotary position thereof, closes some but not all of the openings whereby each of the openings is intermittently opened and closed by rotation of the rotor. And, a bipedal damper includes a pair of legs extending into the flow channels, and integrally joined together at a seat bridging the saddle. However, this region is removed and no problem arises.