a groove on said {HEAD ~ insert wall} positioned such that said extensions are capable of fitting into said groove when said insert body is located within said {TAIL ~ sleeve member} 
a groove on said { HEAD ~ insert wall } positioned such that said extensions are capable of fitting into said groove when said insert body is located within said { TAIL ~ sleeve member }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL