on the centrally disposed boss 29 is provided with a cylindrical outer surface the inner surface 32 is tapered and merges with the semi spherical interior of the recess 33 however at the plane including the top edge 21 of the flange 19 the outer surface 34 of the boss 29 is tapered toward the main body portion 18 i e the tapered walls 23 and 34 converge toward the main body portion 18 where they are intercepted by a bottom wall 36 forming an integral part of the main body 18 the bottom wall 36 having an arcuate inner surface 37 as illustrated in fig3 and 9 the bottom surface 37 constitutes the bottom of the annular channel 38 formed by the integral flange 19 and the central boss 29 and more particularly by the tapered walls 23 and 34 again referring to fig3 and 9 where the tapered wall 34 of the central boss 29 intercepts the arcuate bottom wall 37 of the channel 38 there is provided a circular seal member 39 the base of which is integral with the arcuate surface 37 and slightly radially inwardly displaced from the base of the tapered wall 34 as.