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