Tim Ko's 'War Relics in the Green, ISBN: 962 993 295 4' has a section 'Wanchai Gap and Lady Clementi's Ride'. The map on that page shows three pillboxes. T has already added the one by the dam, and the one to the East of the reservoir. Here's the third.
If you walk down Aberdeen Reservoir Road from Wanchai Gap, a short way down a clear trail leads off to the left. The trail goes straight for a short distance, before u-turning back to the left. From memory, the Pillbox is on that bend.