So I dropped the letter off to the University of Florida's Smathers Library East's Special Collections after speaking with the head archivist there, Carl Van Ness. He said he'll look it over, give it an appraisal, and contact me as soon as he's finished. Until then, here's some more info on the possible writer and recipient of the mysterious letter...
Taken from wikipedia.org: "John Edwin Holmes (1809–1863) was the first Lieutenant Governor of Wisconsin. He was born in Glastonbury, Connecticut. He was ordained a Universalist minister in 1833 and preached for a short time in Michigan and Ohio. He soon began to study law in Illinois and joined the Democratic Party. In 1836, he became a member of Wisconsin Territory's Territorial Council, and in 1848, he was elected the lieutenant governor of Nelson Dewey, a position he held until 1850. After his term ended, he became a quartermaster and was taken prisoner in Tennessee during the Civil War. He was returned to the Union in 1863 as part of a prisoner exchange, but died soon after, in Annapolis, Maryland."
Taken from http://history.rays-place.com/ny/hamilton-ny.htm: "The Dunbar Mills were built in 1850 by James H. Dunbar and Charles Blanchard, on the site of an earlier mill. They are now operated by W. M. Jennings."
Thanks to JJ for the heads up on Dunbar, and for being white.