Mystery concrete block:- (#16 on the layout map)

At first glance it apears to be a anti tank obstacle. its basic dimensions are as follows :-
Top: 48" X 53"
Base: 72" X 72"
Height: 3ft

On the top you can make out where a circular mounting ring was placed, this was probably used to crane this heavy block into place.
There are also two large notches on the right and left side of the block. Obviously something was mounted on this block, but what? Perhaps powerfull naval binoculars? The officer quarters are just above this site.