[poliss]

This website shows the difference between the motors. http://www.tri-ang.co.uk/OO/indexChassis.html
You can still get spares for them. Someone will be along in a minute to tell you where from. Here's one place. http://www.model-net.co.uk/Motor.html

 

[poliss]

The worm of the XO3 in the link I gave is clearly brass. The felt pads came in with the XO4 motor, they didn't disappear.

 

[poliss]

It does appear that there were several variants of the X.03 and X04 motors. The X.04 did not necessarily supecede the X.03 in all cases. Maybe the choice of motor and worm wheel material were model specific. Perhaps there were versions of both motors with brass or nylon worms.
I see the X.04 is also called the Mk.IV motor. http://www.modeltrains.net.au/hornby/servi...asp?sheetid=189
More research is needed I think.

 

[poliss]

The armature replacement you heard about might have been for the Ringfield motor. http://tinyurl.com/58gvcu