The variety Mahan or Mahon is an old heirloom variety. I have also discovered a sweetpotato here in Oklahoma that has been grown by a man for over 30 years which also appears to be identical to mahon.
Another variety that received a patent was "Stokes Purple" which I don't think is a "New and Distinctive" variety as there was no breeding work on it whatsoever.
I hate it when this kind of thing happens! Patents are out of control
http://www.freepatentsonline.com/PP20666.html mahon
http://www.freepatentsonline.com/PP17976.html stokes Purple