Each of the side rail portions of the frame has an upright outer wall and {HEAD ~ an integral horizontal top wall} merging into {TAIL ~ an inwardly and downwardly slanted inner wall} 
Each of the side rail portions of the frame has an upright outer wall and { HEAD ~ an integral horizontal top wall } merging into { TAIL ~ an inwardly and downwardly slanted inner wall }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL