This portrait of McClellan appears in a scrapbook that sets Civil War officers beside a specimen of their handwriting. In this case the portrait is accompanied by a letter from McClellan, written many years after the war while he was Governor of New Jersey, in which he offers his support for what appears to be a widow’s pension request:
The soldier’s name appears to be “Doull,” and there was indeed a single Major Doull serving in the Union Army. Doull seemed to have performed some reconnaissance preserved in this map.
Notes For Bibliophiles 
The wife of Major Alexander Doull petitioned for 20 years for a pension as it was believed her husband died due to illnesses associated with his service. In 1880 congress awarded such pension.