When my father was an enlisted man in the US Marine Corps (1936-1940), he had a high school diploma but most of the men he served with did not. Completing high school was much less common in that era.
I’m guessing the guys that served in the Union Army didn’t have a lot of degrees amongst them.
If your father is still around, tell him Semper Fi from a fellow Marine. If not, know that he is appreciated.