When {HEAD ~ the breech block} is in {TAIL ~ the locked position}, the locking element is supported by the control member and engages the stationary locking surface 
When { HEAD ~ the breech block } is in { TAIL ~ the locked position } , the locking element is supported by the control member and engages the stationary locking surface
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH