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