This
"could be" the addressee of your cover, but you may want to do some more research to confirm.
In quickly searching for "S.G. Hathaway, Jr." of Elmira, NY, I come up with a Lt.-Colonel Samuel G. Hathaway, Jr. of Elmira who was in charge of the 141st NY Regiment during the Civil War. It suggests he was a lawyer and was also affiliated with the firm Diven, Hathaway & Woods of Elmira, NY, was a member of the NY Legislature and was a candidate for Congress (not elected).
This photo is from the Library of Congress Civil War Collection:
