Mynydd Llanyynderyrn Ring Cairn

Surprising that such a fine example of a ring cairn in my own county and not much shouted about, albeit how much is true to its original firm? (I'd like to think most of it).
The obvious anamoly being the large stone at the centre, rumour has it that this may have been a capstone to a cist that was added later after it initial use, as although graves and funery ashes have been found been found in ring cairns, it seems that this was not their original purpose.