Mailbox delivery notifier

Before designing this, I set 4 criteria:

-- The mailbox delivery notifier had to be unobtrusive to both the postal carrier and neighbors--i.e., no wires or visible electronics inside or outside the box vault that could create anxiety.
-- The state of the box contents (delivery v. non delivery) needed to be easily detected by me, but not noticeable to the neighbors or street passers-by.
-- It needed to be reliable, durable, and easy to reset back to an untriggered state.
-- It needed to be inexpensive and low tech, with an uncomplicated mechanism (preferably no strings, springs, or cables)

PHOTOS COMMENTARY:
Underside view of mechanism, with door at right and flag on back of mailbox at left. When the door opens, it swings down on its hinges and pushes in the plunger at far right. This action pivots the pivot rod arm which retracts the flag release latch bar at far left.

You will note there is a short ¼" bolt at the bottom right on the back side of the mailbox, which se