| 摘要 |
The mechanical device is operated by opening the flap (2) or access door (3) connected to a flexible cable (4). The pull exerted on the cable generates a tension in two springs (5,6) of opposing torque and fixed to a locking rod (7). A spring tension (5) greater than the spring tension (6) causes the withdrawal of the locking rod. This movement releases the letter indicator (8), articulated on a fixed axis (9), to pass from a horizontal to a vertical position and visually indicates the presence of a letter.
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