Viscous Fan Removal on 902.940 engine

Just wondering if someone can point me in the direction of how to remove the viscous fan clutch from a a MB 309D with the 5cyl 3 litre 902 engine. Fan
unbolts easy as - 4 allen head bolts, but after that its got me stumped.
There is a metal strip covering what appears to be a small metal button in the centre of the clutch - thats it, thats all that I can find.
My WIS CD does not give details on how to remove either.
Thanks in advance.
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