In both of these embodiments preferably the retaining means includes {HEAD ~ a plurality} of projections projecting axially around the perimeter of one or both of the ends of the cylindrical sleeve and the cylindrical insert includes a plurality of {TAIL ~ apertures} equal in number to the projections located on the surface of one or both of the flanges and located in positions such that the projections fit into the apertures 
In both of these embodiments preferably the retaining means includes { HEAD ~ a plurality } of projections projecting axially around the perimeter of one or both of the ends of the cylindrical sleeve and the cylindrical insert includes a plurality of { TAIL ~ apertures } equal in number to the projections located on the surface of one or both of the flanges and located in positions such that the projections fit into the apertures
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