Those 3 screws show that there is a field-coil bolted to the inside of the cover. A field-coil is an electro-magnet which requires a battery to energize it. Under the cover is also the stator. (And on KZ's there is a magnetic-pole-inverter, which eliminates any brushes or slip rings... very cool, but expensive.)
The other type of charging system uses permanent magnets on the flywheel, (as you said), so there is no field coil, only the stator. This doesn't need a battery to energize it.
