📮 Letterbox on Shameen Island Letterbox Hybrid

Located on the central avenue of Shameen Island, a yellow-façaded house stands among a cluster of colonial-style buildings, serving as a testament to the island’s history as a British-French concession from the late 19th to the early 20th century. This building once housed the Shameen Post Office, which served the community from 1912 until its closure in 1941, following the outbreak of the Pacific War.

You are invited to become a time-travelling postman, journeying through history with a 1920s map of Shameen Island. The map above highlights five significant locations: the historic post office, marked in blue; existing postboxes, marked in green; and a unique letterbox, marked in red. Shameen's streetscape has remained remarkably consistent, allowing this map to bridge a gap of over a century. Follow its clues to locate three postboxes, ultimately leading you to the discovery of the letterbox.

Here's your mission:

1. Collect the letters from the postboxes. Locate the three postboxes labelled "CHINA POST" scattered across the island. Retrieve one letter from each postbox, paying close attention to the underlined letters within the words (e.g. the "A" in "CHINA POST").

2. Sort the letters. The three letters you have collected need to be sorted into different units:

    - Place letters A, B, C and D into Unit 1.
    - Letters E, F and G should be allocated to Unit 2.
    - Letters H, I, J and K belong in Unit 3.
    - Unit 4 will include letters L, M, N, O and P.
    - Letters Q, R and S are designated for Unit 5.
    - Allocate letters T, U and V to Unit 6.
    - Unit 7 will comprise letters W and X.
    - Unit 8 should accommodate letters Y and Z.

Identify which units have received letters, then list their serial numbers in ascending order to form a three-digit combination.

3. Deliver the code to the letterbox. The combination you've derived is the key to unlocking the letterbox. The letterbox is installed on the side of a residential building and belongs to the cache owner. You are welcome to use it. To open the lock, dial in the combination and then rotate the knob anticlockwise.

Inside, you'll find a logbook to record your visit and a special handstamp to commemorate your adventure. There are also unsigned postcards provided as gifts. Feel free to take as many as you like. Please note that the letterbox is currently in use. Kindly respect any personal letters and ensure the letterbox is returned to its original state before you leave.

You've now delivered a message across time.

May you keep in touch with your loved ones.