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