a pair of open channel tracks mounted on {HEAD ~ opposed side walls} of {TAIL ~ the trailer body}, each of said guides contacting an upper surface of one of said open channels, 
of
a pair of open channel tracks mounted on { HEAD ~ opposed side walls } of { TAIL ~ the trailer body } , each of said guides contacting an upper surface of one of said open channels ,
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH