the base having an intermediate, annular groove defined between said ring and {TAIL ~ relief step}, and further wherein said relief step has {HEAD ~ a plurality} of relief recesses defined therein, each relief recess extending radially between the groove and an outer peripheral edge of said relief step 
the base having an intermediate , annular groove defined between said ring and { TAIL ~ relief step } , and further wherein said relief step has { HEAD ~ a plurality } of relief recesses defined therein , each relief recess extending radially between the groove and an outer peripheral edge of said relief step
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