The lower stone lay on top of the upper stone when they were found.
Note the wooden bearing still in the hole of the nether stone and also the rings cut in the top of the upper stone:

The stones are granite, 830 mm in diameter and the top surface of the lower stone is slightly domed:

The matching lower surface of the top stones is chased to take the metal crosshead of the drive shaft which supported it.

The top surface has concentric rings. Decoration, or to help get the grain in?