Who is this person?

[RJNorton]

Kudos, Joe! You nailed it. You win complete freedom from any leftover shoulder pain after your operation last October. We all hope you are doing well and regaining strength.

For many, many years I believed the last photo of Tad Lincoln was taken in Iowa after he and Mary returned from Europe in 1871. A couple of books I own indicated that Tad was very ill and thin for this photo. But all of that is wrong as the boy who is writing is not Tad, but rather it's Mary Harlan Lincoln's brother, William Harlan. The other person's name is not known.

There is an explanation here:

http://lincolncollection.tumblr.com/post...n-identity