{TAIL ~ the extension upper surface} is flush with the upper surface when the extension member is at {HEAD ~ the first position} 
{ TAIL ~ the extension upper surface } is flush with the upper surface when the extension member is at { HEAD ~ the first position }
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD