Yes this is what I was looking at and filmed yesterday on Bodmin Moor near the Cheesewring but found the same photo already in the archives.
http://www.themodernantiquarian.com/site/298/cheesewring.html

From its appearance, and this is only an educated guess as I have not witnessed this in the rain, it appeared that water running off the top stone could run into the 'bowl' on the right-hand side of the stone beneath it. Next to it as you will see is another bowl shaped dugout and between the two a shallow overflow groove had been cut out to allow water to run from one to the other. The one on the left then had a deep groove cut as you can see to the front that presumably was used to run water off into a container/bowl of sorts.
The top stone had a series of what may have been cup marks which one could almost be seen as a pattern. I say 'what may have been' as they were all pretty much identical to each other so I haven't ruled out modern-day workings. How about you other Cornish stoneheads have you seen this or discussed it before?