And this is the reason taken from the BRCA website...
During the last year, there has been much development of the magnetic rotor in these motors. Most manufacturers have changed to using sintered magnet rotors and there are now different size rotors available to be used as a tuning aid. The ruling on the magnetic rotor has been amended to allow the original rotor to be swapped, providing the variants comply with the dimensions stated, have been approved and are available from the manufacturer of the motor. This also means that a motor, which is normally supplied with a bonded rotor, can now be fitted with a sintered version.
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