ion of the apparatus usable in the first embodiment of the invention.FIG. 2 is an exploded perspective view of the first embodiment of the present invention particularly with parts essential to the invention.FIGS. 3A, 3B and 3C are cross-sectional views taken along a line A--A of FIG. 2 in different operative conditions.FIGS. 4A, 4B and 4C are elevational views of a cover locking mechanism of FIG.