We did wonder about the high loss rate of the snapped stones, four of the seven are missing. These stones must have been more firmly socketed as they snapped rather than toppled. Is there something that could make these more attractive to stone robbers?

Presumably these were some of the last stones still standing, is it easier to work a stone when it is standing and then snap it? You would have easy access to both sides, the weathered and the smooth, would that be important?