The middle name of President Harry Truman was just the letter 'S.'

This one is TRUE!!!

John Anderson Truman and Martha Ellen Truman, couldn't decide on a suitable name for their bouncing baby boy, and when the attending doctor finally registered the child's birth with the county clerk a month later, the infant still had no name. Eventually the Trumans chose to name their boy "Harry" after his maternal uncle, Harrison Young. Unable to decide between a middle name honoring Harry's maternal grandfather (Solomon Young) or his paternal grandfather (Anderson Shipp[e] Truman), John and Martha opted not to give little Harry a middle name at all and settled on something that could represent either grandparent: the letter 'S' by itself. (As Truman biographer David McCullough noted, using a single letter that stood for nothing specific was "a practice not unknown among the Scotch-Irish, even for first names.")

Although the 'S' was not technically an abbreviation and therefore did not need to be followed by a period, Truman's full name was generally rendered as 'Harry S. Truman' during his lifetime, and Truman himself used letterhead bearing the name 'Harry S. Truman' and signed his name with a period after the 'S,' as shown in this excerpt from a letter on file at the Truman Presidential Museum & Library: