Each side rail portion includes {HEAD ~ a horizontal wall} between {TAIL ~ the inner wall} and the mid wall 
between
Each side rail portion includes { HEAD ~ a horizontal wall } between { TAIL ~ the inner wall } and the mid wall
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH