The caption in the Webster edition is not in italics in either of the two
1885 printings, nor the 1886, 1888, or 1891 reprints. I think the 1891
edition was the last printed from the original plates. So, no, it has no
bibliographical significance, but it could have some textual significance if
there is evidence that the change represents an authorial change. Have you
checked the 1896 (issued as part of the aborted Harper collected edition),
1899 (Author's edition), or 1903 (Hillcrest) editions?

I got my count on "nigger" in HF from doing a search on the Twain's World
CD, and I remember that I was doing that to verify somebody else's count,
but I don't recall whose. I don't recall if I counted contents,
illustrations, captions, but I'd assume that I did.

Kevin Mac Donnell
Austin TX