In Wm Borlase’s Antiquities of the County of Cornwall, pub. 1769, the inscription is read thus:
CIRVSIVS HIC IACIT CUNOWORI FILIVS
The reading of the first name as DRVSTANVS seems to be generally accepted at present, with the initial letter(s) construed as a ‘D’ written back-to-front. I have never yet been able to find the evidence upon which such a reading is based: for example, other inscriptions from (around) the same period in which a ‘D’ is written back-to front. Do such examples exist? If so, where?
In fact the "Drustanus" reading is a modern one, proposed by a small group of scholars that included the archaeologist Raleigh Radford and the Arthurian romance specialist André de Mandach. They saw the stone as a crucial proof of the historicity of the legend of Tristan and Isolde. No earlier visitor read the inscription as a "d", even when it was much easier to read than it is today. The earlier consensus was that it read CIRVIVS.
More in the afterword to this:
mikedashhistory.com/2015/12/28/the-breton-bluebeard/