| 摘要 |
The signal (7) is held in a horizontal position by a foot (2) which extends from the side of the flap (3) of the letter box. When flap is opened the signal is released by is brought to a vertical position, above the box, by a counterweight (4). The signal is fixed to one end of a lever (1) and the counterweight, or a spring, is joined to its other end. The foot is at an acute angle (a) with respect to the vertical so that it rests on the signal.
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