Here is a picture of the original from the attack disk manual. Inside the top cover there you can see the piece that can be placed on top of the compass repeater. The round disc there with the markings is the target course disc with AOB marks just like on the attack disc. There is also a moveable bearing arm as well as an attack course arm, just like on the attack disc itself.
So this effectively turns the compass repeater into an attack disc.
Here a closeup of a real compass repeater on a boat - 180 on top:
