The blog linked to an img of a newspaper from the period. Here is the Library of Congress's archive of the same paper, showing the same letter. Now, that doesn't show that the Tribune wasn't duped as to the letter's source, but it does show that the grammar is from that period. Assuming you trust the LOC.
http://chroniclingamerica.loc.gov/lccn/sn83030213/1865-08-22...