Found it! Quite why I couldn’t find it before, I’ve no idea. The stone is 27 paces at a bearing of 120 deg. from the modern granite pillar inscribed with the number 7 (a waymarker for the Llanfairfechan Upland Walk). The markings can be clearly seen along one edge and on the surface of the stone. Somehow, perhaps because of the short lengths of the markings, I find the antiquity of this stone less convincing than that of the other arrow stones in the vicinity.
The stone marked with concentric squares is another 12 paces downhill at a bearing of 20 deg. from the arrow stone. It lies in a direct line with the signpost at the path junction and the granite pillar