The second O-ring is for use with the later oil pump/cases, that have twin feeds. Look on the underside of your pump, and see if there is a secondary hole (about 2mm in size).
Late MC21s* and all MC28s have the second feed. MC16, MC18, and early MC21s have just the single feed (and therefore just the one large O-ring.
*It's not as simple as just a change of model year, so don't ask when they changed! Just look at the underside of your pump and the crankcase for two holes!
Note: The MC28 parts manual illustration more accurately depicts the second O-ring.
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Andy.
NSR-WORLD.COM
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