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