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