Some superficial research at the NMRW clarified the history of this site, and made me feel a lot better about not finding it. According to the Royal Commission’s records...
This stone, known as Coitan Arthur, Arthur’s Quoit – probably the capstone or supporter of a demolished cromlech – stood in the corner of a field appurtenant to the farm of Trellwyn Ucha. It was destroyed in the year 1844.
I’d like to thank Phil for adding every site with the word ‘Arthur’ in it that he could find to the Modern Antiquarian Web site, including this one which hasn’t existed for 159 years!