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
I couldn't find anything either, I was walking around in the pouring rain trying to find the stone for at least half an hour before heading home.
Sanity prevailed in the end. I'll try another day, when it's sunny.
I could find nothing at this location although both Coflein and the Gwynedd Archaeological Trust list an arrow stone at SH 6937 7228.
I also couldn't find the stone from the Roman period which the GAT list at SH 6936 7231 and which Coflein describes as: "Incised stone at the head of Alan Glan Sais near the Roman road. The surface of a flat rock 3ft 8ins by 2ft 5ins bears a design consisting of three concentric squares, the centre of each of the four sides of this design being cut and measures about 9ins square."
"Some Arrow Stones and Other Incised Stones in North Carnarvonshire and North Denbighshire" by W. Bezant Lowe in Archaeologia Cambrensis 1924 pp. 340 - 362 gives the following description:
"Situated 100 yards NNE of the signpost, and the direction of its greatest length is NNE by SSW. Its length is 9 ft. 6 in. and its breadth 3 ft. 2 in. The edge is quite straight and the surface of this side slopes at an angle of 20 degrees from the vertical." He describes 10 "more or less vertical marks" on "the upper edge of the sloping surface i.e. along the edge" and 29 markings in 7 groups on the top flat surface.
The nearby stone incised with concentric squares is described as "about 110 yards NE of the signpost, flat and level with the surface of the ground and partly covered with turf and moss. It is an irregular square, the sides measuring roughly 3 ft. 8 in., 2 ft. 6 in., 2 ft. and 2 ft. 6 in. respectively."
The signpost is said to stand at the meeting of 4 tracks, which I presume is at SH 6932 7223.