I'm not an engineer, but I assume that the centre seal stops pressure leaking across each half of the crank case. So from the picture in the link is shown as the broken bits
http://www.nsr250.net/forums/viewtopic.php?t=13726&highlight=center+seal
I'm guessing the locating pin and rubber ring ensure the bearing doesn't spin and provides a minimal seal for the outside of the bearing which will already be sealed against the crank cases
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