A ledge or mandible engaging support surface is defined between {TAIL ~ the upper tier} and {HEAD ~ lower tier} and terminates in an exterior opening 
A ledge or mandible engaging support surface is defined between { TAIL ~ the upper tier } and { HEAD ~ lower tier } and terminates in an exterior opening
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH