What you have is die deterioration doubling. http://www.error-ref.com/?s=die+deterioration+doubling++ If you want to look for errors I suggest using that site.
You can also use these sites: http://doubleddie.com/ http://varietyvista.com/ http://lincolncentresource.com/ Also, if you don't have one, buy a copy of the Red Book. Official title is "A Guide Book United States Coins" by R.S. Yeoman, published by Whitman. It's the book every collector should have. Loaded with so much useful information.
