Cheryl Straffon's guide 'The Earth Mysteries Guide to Bodmin Moor and North Cornwall (including Tintagel)' (Meyn Mamvro - 1993, amended 2000) reports a "small menhir or markstone at SX12537258. Walking towards this mark stone on the top of the rise the Colvannick Tor Stone Row comes into view across the A30".
Easy to spot as it lies in an area of the Downs with little bracken and few stones. However it is extremely small and very reminiscent of two boundary / marker stones I saw on the Downs, and the pitiful Peverell's Cross. It has about 60cm of stone above ground, with an extra 25cm as part of the 'pit' it stands in. However, from my memory this stone doesn't have any letters carved on it, unlike the boundary markers elsewhere on the Downs. Curious.
Elsewhere in the Straffon booklet she writes that Trehundreth Downs Menhir is aligned to a cairn, and this stone. Add to this the Colvannick Tor Stone Row and "evidently all these stones were part of a special alignment, and perhaps a spirit path of the dead associated (sic) with the burial mounds here". Maybe Straffon wants this to be a menhir, rather than a boundary stone, because it is conveniently aligned to the stone row?