I've only seen this one instance in the Barrett Collection of MT writing out all five of his names in full, and no instances of him using "Samuel Langhorne Clemens." Tom Sawyer would have called that a name to lick someone by. MT is using it here to sign a manuscript of the toast "To Woman" he delivered in 1882, at a dinner for the New-England Society of New York. The date "1891" indicates he's making a present of the MS. long after the speech, but there is no record of to whom, or any indication why in this case he used "Langhorne."