This stone situated in a shallow gulley at Knack Mine (Steeperton Tin mine) has a surprisingly rather complex purpose in marking the bounds of ground for mining purposes. It can't be stated any clearer than on this excellent website - please click on link if you would like more information.
(WV Stone link)
The stone has the letters WV carved in one side.
This stone is within the Okehampton live firing range so please consult live firing notices.