In this particular embodiment, the seating mechanism 8 includes a protrusion or ledge 8, i.e., a sealing surface 8, positioned inside the cover assembly which frictionally mates or seats adjacent a distal edge, lip or ledge, i.e., a sealing surface, of a base assembly such as sealing surface or edge 15 of base assembly 6 of FIG. 5, herein shown as a protrusion or ledge 8 which encircles the inside