This mechanism assembles to the trough in Williams games from System 11 through WPC-95. When the playfiled is lifted, the flag blocks the ball trough and stops the pinballs from bombing the playfield. Installation takes about 15 minutes, and requires removal of the apron.
The assembly is made from stainless steel, and is thin enough that it can fit underneath playfield obstructions under certain games. The flag is deigned to hang correctly on a 6.5 degree playfield so that it does not interfere with ball eject.
Early WPC installation requires that the ball guide be bent back to make room, for the ball stop. The ball stop covers the space opened.