Hugo, I am guessing that as you had a very recent walk starting and ending in Penmorfa ( "Top of the sea marsh?"), and the fact that there is a King George's Playing Field in that village is leading me towards that location, but I have no idea what lies below the inscription.
Incidentally, there are some programmes on S4c this week between 2000 and 2300 on Tuesday, Wed, Thursday and Friday which might be of some interest to you and your walking companions about climbing. One of them features Ioan Doyle from Bethesda and some of the others feature Eric Jones who has a café on the road to Beddgelert below the cliffs in Tremadoc, and the rest are about Everest and one on the "Dragon's Back Race".