I remember my gran singing this and I think I have some more lyrics. I think the song starts like this-
In a factory in a city an explosion once took place
and a girl so young and pretty lost her sight in that sad way
made her think she'd lose her lover
a soldier (or poss sailor) boy so young and brave
so she wrote to him this letter
telling the state of her sad fate...........

Then you get the verse as above about Jack dear. Unfortunately I can't get past this although I seem to recall that he takes the letter to his commanding officer to ask for leave and goes to visit Grace, explains that he would never leave her and therefore we have a happy ending. Can anyone expand on this.