The Armature plate / steel disc is designed as a opposition for electric holding magnets with a diameter of 80mm.
But is of course suitable for other articles and purposes as well. Such as magnetic-field blocker/reinforcer for magnetsystems or permanentmagnets.
Place the magnet on the disc to "push" the magnetic field. This way the magnet force gets stronger, however the field gets shorter.
The hole in the middle is countersunk for mounting.