I'm hoping it'll look like this - but bigger.

http://www.themodernantiquarian.com/image.php?image_id=10778

There are some large stones with big holes. Men-an-tol is the obvious one but there's a better example in Derbyshire called the Devil's Ring (and finger). It seems that these big stones were used as a portal inside a chambered tomb. This little stone, in Kirkhaugh, was probably used to bung that hole in the larger stone closed. The sizes match and so does the angle of the bevel.

(This is my latest crackpot theory).

I did suggest that soeone could take the stone to Cornwall, to test the theory, but now, with the Devil's Ring, the journey need only be half as long.

Bosophrennis also seems to have a tumulus described as being quite similar to one on Bolton moors, but three or four times larger. There was some ancient antiquarian speculation that the Cornish tumulus, with a deep hollow on top, had held a small cromlech. That would make for an interesting reconstruction - there's quite a bit of free stone about (and so on)

david