end of the assembly formed by the sleeve halves 24 and 46 projects beyond the lower end of the upper casting 36 and a peripheral retaining ridge 72 encircles the lower end of the assembled sleeve halves 24 and 46 with the parts so assembled the outer bottom sleeve half 22 and the inner bottom sleeve half 42 can be disposed to encase the lower portion of the upper casting 36 as shown in fig1 a and 11b the outer and inner bottom sleeve halves 22 and 42 have inwardly facing retaining channels for matingly engaging and longitudinally retaining the peripheral retaining ridge 72 of the lower end of the assembled sleeve halves 24 and 46 with that when the outer and inner bottom sleeve halves 22 and 42 encase the upper casting 36 the lower ends of the core sleeve halves 24 and 46 will be fixed against relative longitudinal movement while the upper casting 36 will be permitted to slide longitudinally upon an actuation of the trigger handle 30 and the underlying trigger 54 in a further assembly stage as shown in fig1 the assembly formed with among other components the outer and inner bottom sleeve halves 22 and 42 can be slid longitudinally into the lower casting 44 through the open upper end thereof until upper.