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From this article, here's the measurements of the clutch basket rubbers on a Honda CX500 CDI model ('78-~'81). First, the shape of the hole:
All of the rubbers measured between 11.20 and 11.34mm in height:
Width around 27.88mm:
As far as the widest point of the holes go, 19.15mm (should go and measure the post too):
There's also a slight lip on one edge which you can see here:
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