Harry Sergeevich Truman

Once in the early '90s in an American teen comedy I heard a curious fact. But first a little bit about the movie plot. Simple and dashing grandfather and his deadhead grandson exchange their bodies for a while. And a mischievous old man solves some problems for the grandson. And there is a moment when the grandfather tells the "little known" fact of Truman`s life. That S. in his name means Sergey - seems to be that his mother loved Russian literature and identified her hobby in that way.
Then I almost believed it, but was never able to check. Now I found out that S. is initial. Names of both Truman grandfathers began with S.