Pssst...I'm considered a research specialist. Everytime I see this type of "History Mystery" item....prepare for the inclusion of the item in a new book.
Probably one around there ~ which is why they cut the stone on the other side of the river and further North. Somebody figured they'd made a mistake on latitude and they attempted to correct.
This is NOT an 18thcentury British survey ~ but something far older ~ back when they'd actually use a magnetic compass in a land survey.