An apparatus 4, wherein said pocket inner surface of said retainer is formed into a shape which substantially coincides with the shape of the ball in contact with said pocket inner surface at {HEAD ~ the major axis position} of {TAIL ~ said cam} 
An apparatus 4 , wherein said pocket inner surface of said retainer is formed into a shape which substantially coincides with the shape of the ball in contact with said pocket inner surface at { HEAD ~ the major axis position } of { TAIL ~ said cam }
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