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@moth
The armature probably includes some magnetically-soft iron to provide a low-energy path for the magnetic field lines to get between the north and south poles. Take this away and the same amount of field passing through air would have a lot more energy - more than the magnets can withstand, so instead they will re-arrange the magnetic fields within themselves to mostly cancel out, and so become much weaker. A keeper is a piece of soft iron placed between the poles of the magnet before or as the armature is removed so that there is always a low energy, high permeability path for the field, thus preventing the magnets becoming unstable.
(As you work against the magnetic field to pull the armature out of the magnet you add energy to the field. Use a keeper and the armature will be removed with much less force - the keeper being pulled in and helping to push the armature out.)
With the hub in the state shown (left-hand cone removed) the next step in dismantling it is to remove the thick spring wire circlip holding the sprocket on by prying at the three semi-circular notches.
To unscrew the right-hand-side of the hub (K60 in BlueQuinn's diagram) from the shell, apply rust penetrating oil to the join and leave overnight, then clamp the shell in a vice and tap sharply at the notches in the ball ring with the shaft of a cheap screwdriver and a hammer. The ring unscrews anti-clockwise, looked at from the right.
moth