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On a tangent to this Tomb is always quite proud of a set of valve covers from a company called Blake, if I recall..A bit taller with additional finning, the inside was indeed layered with some heat/oil resistant thick coating (rubbery) to reduce the tappet noise..
Maybe he used the rivet to plug a hole from a previous attempt at what he was trying to accomplish.
I would say that is as good a guess as any. I believe the "Tips" book said a feminine napkin would quiet them down.