said lengthwise movement of said screw lock member extending to an engaging position whereat said {HEAD ~ key pin} engages with said recess and to a non engaging position whereat said recess disengages from said key pin thereby permitting {TAIL ~ free rotation} of said lead screw; and 
said lengthwise movement of said screw lock member extending to an engaging position whereat said { HEAD ~ key pin } engages with said recess and to a non engaging position whereat said recess disengages from said key pin thereby permitting { TAIL ~ free rotation } of said lead screw ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH