inner wall of the flat top 106 near the end 108 are two downwardly extending concentric annular bosses 116 118 the boss 118 being closer to the top 102 and being larger in radius than the boss 116 it should be here noted that there is substantial separation between the boss 118 and the inner wall 120 defined by the end of the section 114 for purposes presently more fully to appear centrally located on the top 106 is a bore 122 and located on the outer portion of the top 106 are two upwardly projecting stops 124 126 the lower housing 200 similarly comprises an elongated shell 202 which is provided with an enlarged end 204 an elongated flat bottom 206 and an annular wall 208 at its other end the enlarged end 204 is provided with three concentric annular sections 210 212 214 extending inwardly from the end 208 to the center of the housing 200 the section 210 being the largest in radius and the section 214 being the smallest in radius provided on the inner wall of the flat bottom 206 near the end 208 is an upwardly extending annular boss 216 provided with a concentric bore 218 which extends through the boss 216 and the bottom 206 it should be here noted that there is also substantial separation between the boss 216 and the inner wall 220 defined by the end of the section 214 for purposes presently more fully to appear centrally located on the.