When cocked, it rest on {TAIL ~ the tab} formed on {HEAD ~ the flange} of a backwardly flanged mail door 
When cocked , it rest on { TAIL ~ the tab } formed on { HEAD ~ the flange } of a backwardly flanged mail door
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH