Originally known as the Tytam Intermediate Reservoir, it was constructed between 1904 and 1908 according to PWD Reports of those years. Additionally, "for dam safety reasons, the north section of the spillway has been lowered in 1977, resulting in today's indented appearance of the crest of the dam" ( www.aab.gov.hk/form/AAB138-14-B-eng.pdf ).
There was a pumping station at Bowen Drive, just below Bowen Road, next to the Filter Beds. Not sure when it was built, but it doesn't appear in early pictures of the BMH. It was demolished in the early '50s, when the Service Reservoir was rebuilt.
Over the entrance, it says 1965.
Street view here.
Klaus found this tank is located a short distance noth-east of number the house "Trollstua" at 8, Barker Road.
It is marked on government maps as a Fresh Water Service Reservoir.
Timeline:
This sits over the entrance to the tunnel that carries the water from the reservoir to the north side of the island. It was built to keep a record of the volume of water flowing through that tunnel.
Notes from the WSD leaflet about the Tai Tam Waterworks Heritage Trail: