Each one of the at least one pair of {HEAD ~ guide rails} is positioned on a different side of {TAIL ~ the interior surface} of the collar 
Each one of the at least one pair of { HEAD ~ guide rails } is positioned on a different side of { TAIL ~ the interior surface } of the collar
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH