by a first annular notch reduced diameter portion in the lower outer surface of the sleeve the upper portion 13 has a second shoulder 13 c formed by a second annular notch reduced diameter portion in the upper inner surface of the sleeve the lower portion 15 of the cap comprises an l shaped ring having an axial extending component 15 a and a radially extending component 15 b the axially extending component 15 a abuts and is fixed to the first shoulder 13 a of the upper portion 13 of the cap the overall outer diameter of the ring is equal to the outer diameter of the upper portion 13 of the cap thereby forming a flush outer surface the screw 16 is rigidly affixed to the upper portion 13 of the cap in a preferred embodiment the head 11 of the screw 16 sits in the second notch with the underside of the head contacting the second shoulder 13 c and the annular rim contacting the reduced diameter portion 13 d of the notch the cap has a downwardly extending skirt that surrounds a retainer the axial component 15 a of the lower cap portion is of sufficient length to form an internal gap u201c g u201d between the bottom of the upper portion 13 of the cap 14 and the radial component 15 b of the lower portion 15 after the two cap portions are joined this gap creates a groove 19 along the inner wall of the cap in which portions of clip 18 are seated as depicted in fig5 the axial component 15 a of the lower portion 15 defines the radial boundary of the groove 19 in a preferred embodiment the clip 18 is non circular and has.