Several people have suggested explanations of what the stone is/was. Most agree it is was not a corner stone.
Some have suggested it was a millstone for grinding grain, possibly an unfinished one. Others say the hole in the middle is larger than millstones would usually have. Another suggestion is a well capstone. The hole in the middles may not be big enough for much more than a small bucket to fit through. Another suggestion is a vertical crusher stone. See picture attached.
Earl Taylor
Dorchester Historica Society