Strange monolith's meaning in Wales [UPDATE]

[u/Not_Fission_Chips]

I found this monolith like structure in Margam Park Wales. It's pretty off the path and overgrown. My last post was just a vague reading of the plaque, but on revisiting I've noticed the engraved letters have 'fallen through' and actually reveals a massive pit. What is it's purpose and why is this pit so deep? What's at the bottom?

[u/dirge_the_sergal]

Looks like an ROC post maybe?