As the site listings and comments describe, the stones at Mystery Hill are a real jumble, with substantial evidence that earlier owners moved stones to where they thought they might have stood originally, or where their locations might support the 'Irish Monk' theory.
One of the most fascinating stone's is the great slab of the "Sacrificial Stone." About a year ago I was looking for antique hinges and found Old Wood Workshop, and eventually: this lye leaching stone. Although I am more familiar with leaching lye in a wooden vat like the one on this page, stones where also packed with ashes and a carved runnel carried the lye water to a waiting bucket. (This would possibly work even better because the slower the runoff, I would think the solution would be stronger.)
It turns out that this question (colonial lye leaching or Native American manufacture) has already attracted the attention of people examining Mystery Hill. See the site Lye Stones, Cider Press Stones, & Native American Grooved Stones and Mary E. Gage's article on examination and conclusions of New Hampshire state archaeologist, Gary Hume. As referenced in some of the comments made previously, Hume believed much of Mystery Hill's stones are Native American constructions, and Gage quotes him on (if I am following this correctly) other grooved stones at the site, although not this particular one. (Which, I must say, looks like the other lye and cider stones described in the http://www.stonestructures.org website.)
A great deal of Mystery Hills authenticity as a pre-colonial site depends on how much the owner Godwin moved stuff around in the 1930's and 1940's.
Something is not right. This message is just to keep things from messing up down the road