In Borlase's Antiquities of Cornwall, pub. 1679, the inscription was read as:
CIRVSIVS HIC IACIT CVNOWORI FILIVS
Who was responsible (much later) for reading the first name as DRVSTANVS? Initial letter intepreted as D written backwards, not as CI. I have always been highly skeptical about that!
Since I was a child, with grandparents living in Fowey, I have been fascinated by this longstone, and the site of Castle Dore.