Tomorrow is Forever
(1946)

Erik Kessler (Orson Wells) never truly admits that he is John Andrew McDonald, though there are stong indications he is.
He tells Elizabeth Hamilton (Claudette Colbert) that the past is gone.

Erik convinces Elizabeth to let Drew, their son, join the Air Force.
Erik dies and Elizabeth and her husband adopt little Margaret (Natalie Wood).
A half burnt letter lies in the fireplace. We don't know what was in it.

Thanks Erika!