‘The Shire Stones’ are marked on John Speed’s 1610 map of Gloucestershire and three stones are clearly marked. They appear to be a little to the north by the junction with the Fosse and the road to Cold Ashton, and widely spaced, separated by the roads, although this might be artistic licence on the part of Speed. Nevertheless, there were certainly three marker stones there over a century before 1736. Either those engraved with 1736 are the stones that were there in 1610, or they replaced those stones.
Digital copy of the map here: upload.wikimedia.org/wikipedia/commons/1/1c/John_Speed_-_Map_of_Gloucestershire_-_1610_-_001.jpg
