the signal device is free to fall by gravity into {HEAD ~ a vertical position} when triggered by opening {TAIL ~ the mailbox door} 
when triggered by opening
the signal device is free to fall by gravity into { HEAD ~ a vertical position } when triggered by opening { TAIL ~ the mailbox door }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL