Hi Roy ,there has been some limited excavations in this country around marked boulders that are not bed rock (be rock ahs been excavated too ) but none have been inverted unless they were small enough .In the cases of genuine standing stones still standing with markings there are examples where both sides have been marked .The sequence is intriguing enough when the markings are on one side i.e. was the rock marked in situ then quarried and erected , the most likely sequence , or quarried then marked .When the markings are on both sides ,(sometimes the sides of the stones too therefore 3or 4 sides as is the case at Nether largie ) clearly on one side could have been done in situ but the stone could then have been turned over .