196 1/2 feet and 8 degrees west of north from the nearest stone of the circle is a stone 5 feet 4 inches high, with a hole 5 1/4 inches in diameter through the upper part of it; this stone is now used as a gate-post and may perhaps not now occupy its original position..
An old stonebreaker, who told me in 1898 that he had been in the place for seventy years.. said with regard to the holed stone, that it had been moved from its original position, where it had stood in connection with another holed stone, and that when the sun shone through the holes in some particular way "they called it Midsummer"; this may be only a repetition of something said by modern visitors, but it may, on the other hand, be an echo of an old tradition, so it is perhaps worth recording.
Prehistoric Remains in Cornwall
A. L. Lewis
The Journal of the Anthropological Institute of Great Britain and Ireland, Vol. 35. (Jul. - Dec., 1905), pp. 427-434.
I'm guessing this could be the stone mentioned below, and it has indeed been moved as Phil suggests. I'm not sure the sizes fit, but the hole might look suitably wide. Otherwise, I guess it's a different one, holed stones galore.
Near the Dawns Men, in the fence of a field by the footpath leading to Borah, is another holed stone, laid down lengthways. It is thirty-three inches wide at one end, tapering down to ten inches at the other, and seven inches in thickness. The circular hole, five inches in diameter, is seven inches from the extremity of the widest portion.
"About 65 paces exactly north of Rosmodreuy Circle is a flat stone 6 inches thick at a medium, 2 foot 6 inches wide and 5 foot high; 15 inches below the top it has a hole 6 inches diameter, quite through....In the adjoining hedge I perceived another, holed in the same manner."
William Borlase - Observations on the Antiquities, Historical and Monumental, of the County of Cornwall - 1st edition, printed by W Jackson, 1754.