The Gettysburg Address and Bixby letter was a commenoration piece done on the anniversary of Lincoln's birthday. The letter is to Mrs. Bixby who lost 5 sons in the war. (My own great grandmother lost 4 sons in WWII)
Civil War-era letter from Abe Lincoln was written by his SECRETARY
Using the 'N-gram' technique, researchers from Aston University and the University of Manchester determined famous Bixby letter was 'almost certainly' written by secretary John Hay.