The stopper mechanism controls and releases {HEAD ~ retraction} of the retractable unit to {TAIL ~ the closed position} in which the unit is retracted, and includes a stopper and a blocking member 
to
The stopper mechanism controls and releases { HEAD ~ retraction } of the retractable unit to { TAIL ~ the closed position } in which the unit is retracted , and includes a stopper and a blocking member
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH