{HEAD ~ 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 {TAIL ~ contact} with said pocket inner surface at the major axis position of said cam 
{ HEAD ~ 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 { TAIL ~ contact } with said pocket inner surface at the major axis position of said cam
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