These sound really neat!  You might want to try contacting your local historical society or museum.  They will likely be very helpful and perhaps knowledgeable about the agricultural past in your region.  Your best bet is to take pictures of them, and make a little map of where you found them.

I would think such a specific item would go to a specific collector - but you could use them foot stones on a garden path, or even donate them to a local historical museum.  There is a house in my neighborhood where a huge millstone is the centerpiece of a garden bed - it's awesome looking!   I hope to see pictures of them on iTaggit soon!  

I'm a collector of old millstones and milling equipment...and would be very interested in seeing the stones you uncovered.....possibly buying them from you if you want to sell them.  Please contact me at    wills3@earthlink.net

Thanks