Iron water tank at top of Hong Kong University campus [????- ]

In the 1945 photo of HKU posted recently, an 8-sided roof down in the foreground caught my eye:

It's the roof of a cast-iron water tank. The tank is right at the top of the campus, near the entrance to the university from Kotewall Road.

I've seen it many times over the years, and wondered how old it is. Now we know it dates back to 1945 at least.

Here are some photos I took this morning. If you walk along the path above University Avenue, when you get near the top you'll see the tank off in a clump of trees to the left:

One of the panels has the text "B O" and a symbol on:

The roof has rusted through. Poking a camera inside shows the old roof supports still in place, but that several of the roof segments have collapsed:

Can anyone tell us when it was built, and what the characters and symbol mean?