The upper inside surface and the lower inside surface of {HEAD ~ the housing} define a group of {TAIL ~ slots} 
The upper inside surface and the lower inside surface of { HEAD ~ the housing } define a group of { TAIL ~ slots }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL