This can be found in a church alongside the road to Burwick pier where the short sea ferry crossings take place in the tourist season). It is now in a back room at the church, and the key can be had from the lady who runs the local P.O. As this is on the 1:25,000 but unsignposted it is probably best to enquire at the Tomb of the Eagles in order to avoid disappointment outside of its limited post office hours.
Saint Magnus appears to come into the story sometime between the 16th century and 1690. Down in Caithness a story was told of the saint turning a dragon into the Stone Hone in the Watten parish, now reduced to rubble.
The first mention is that a man was shipwrecked and got home by jumping on the back of a monster, which he then turned into the stone, this "Gallus" promising to dedicate a church to St. Mary. Although the writer wrote of the stone being by a 'temple' near the shore I presume this simply means the kirk. Between this time and 1701 it seems to have been reduced from 6'x4' to the present dimensions - perhaps this was to remove some pagan feature.
By Gallus we might be dealing with another word for a RC priest - perhaps Magnus' name was attached to protect the stone at a time of religious upheaval. Of course as this saint was an earl of virtually kinglet status it could have been attached even earlier.
Previous chapel is grassy mound between on banks of Burwick loch, between this and and shoreside road. X for St.Colm's Chapel shown on 1882 map at ND44168427. Originally occupied an islet in the now-drained loch so probably on a crannog/broch/island dun site.
In the kirkyard of St.Mary's Church near the Burwick pier there is a water-worked whinstone with two hollows resembling feet .
It has been suggested in the last few years that it is the inaugural stone of a sub-king with land in both South Ronaldsay and Caithness.