I’ve been asked to post this to see if anyone can identify this structure.
The information I have been given is that it is not a pillbox, there is no access, no windows and no interior. Its about 3 feet high and 4 feet across, with a threaded metal rod in the centre top. There is also a triangular depression in the top, maybe tripod leg locators.
Suggestions include a small searchlight or rangefinder mount, a spigot mortar mount, a ROC observer post or something to do with a decoy site.
There are no other buildings nearby and it is on top of the highest point in Marchwood enclosure, beside the A326 (west side of the road to Ipley Crossroads)