{TAIL ~ The mid wall} has a longitudinally extending upper boss and {HEAD ~ a longitudinally extending lower boss} forming an outwardly open slot therebetween facing the outer wall 
{ TAIL ~ The mid wall } has a longitudinally extending upper boss and { HEAD ~ a longitudinally extending lower boss } forming an outwardly open slot therebetween facing the outer wall
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH