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