Ha!
@Mike Serpa actually posted a likeness of
Charles Andrews Snow 3rd MA on his FindAGrave memorial and it is the same pose. Mike cited the copy of the image to "The Third Massachusetts Regiment Volunteer Militia in the War of the Rebellion, 1861-1863," John G. Gammons, 1906
So yes, indeed, we can now say with 100% certainty that the "handsome Yankee" is
Charles Andrews Snow (12 May 1829 - 28 November 1903) who served as Chaplain of the 3rd Massachusetts. Don't you love it when we can positively ID an image whose identity has been temporarily lost?
Thanks so much for everyone's kind assistance and to Mike Serpa for posting the image from the book at FindAGrave.