I don't know about the penknife connection. The story as I heard it was that a stranger approached Lincoln and informed him that he had vowed to himself to kill the first man uglier than himself. When he met Lincoln, he felt he had found that man. Poor Lincoln.
And speaking of Hal Holbrook. I think I heard he has a role in the new Lincoln movie coming out this November. Of course, he is not playing Lincoln this time but Francis Blair.