A second monoblock element has the shape of a circular cap with a flat blind end and covers the first element so that said blind end of the concavity is in contact with the horizontal plane situated at the top of the cone frustrum, and {HEAD ~ the horizontal peripheral edge} is in contact with {TAIL ~ the rubbing plate} 
A second monoblock element has the shape of a circular cap with a flat blind end and covers the first element so that said blind end of the concavity is in contact with the horizontal plane situated at the top of the cone frustrum , and { HEAD ~ the horizontal peripheral edge } is in contact with { TAIL ~ the rubbing plate }
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