This was one of 'Technical Failures' in our days of visiting such places, still being in official hands at the time, and in use for training. Made it to the gates of the site, but not inside.
The terminal block is extremely non-standard. and we've seen nothing like on any of the 200 posts we've visited.
It's not classed as a Master Post, which would have had and additional connector system referred to as a 'Dome', for the wiring to an extending mast that would have been clamped to the vent. These are very distinctive and easy to spot - and the posts are also covered with fairly heavy earth strapping, similarly obvious and easy to spot.
At a guess, it was probably something added to allow direct communication between the post and the adjacent bunker.
The normal GPO (or latterly BT) comms wiring would have been run underground, straight into the post.
Some posts had overhead lines, but most show no evidence of the line at all, other than inside. But sometimes it has been ripped up near the surface, after they were stood down.
Interesting additional cover, possibly as you say from a new owner as the post was to be put on the market in 2011, but no further info. Although it was noted that the original locks etc were in place.
Thanks... and welcome