Letterbox on Shameen Island Letterbox Hybrid

Located amidst the verdant foliage of Central Avenue, a magnificent house with yellow facades stands among Shameen's array of colonial-style architecture, serving as a reminder of the island's past as a British-French concession during the late 19th and early 20th centuries. This house was in former times Shameen Post Office, offering its services from 1912 until its closure in 1941 due to the outbreak of the Pacific War.

For your exploration of Shameen Island, you can rely on a historical map dating back to the 1920s. This map highlights five significant locations: the historic post office marked in blue, current postboxes marked in green, and a unique letterbox marked in red. Remarkably, the island's streetscape has endured the passage of time largely unaltered over the last century. Please use the old map as a guide to locate and examine the three postboxes, ultimately leading to the discovery of the letterbox.

Follow these steps to accomplish your mission:

1. Collect the letters from the postboxes. On the island, you will find three postboxes, all bearing the label "CHINA POST". Select one letter from each postbox, noting the underlined letters within their respective words (e.g., the "A" in "CHINA POST").

2. Sort the letters. The trio of 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 should accommodate 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,
    while Unit 8 will include letters Y and Z.

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

3. Deliver the code to the letterbox. The combination you've derived serves as the key to unlocking the letterbox affixed to the side of a residential building. This letterbox belongs to the cache owner, and you are welcome to use it. To open the lock, please dial in the combination and then rotate the knob counterclockwise.

You will find a logbook and a handstamp inside the letterbox. Moreover, some unsigned postcards are provided as gifts for you. Please take as many as you need. It is important to note that the letterbox is currently in use. Kindly respect any personal letters and ensure that the letterbox is returned to its original state before your departure.

May you keep in touch with your loved ones.