Donna Campbell writes that she found the epithet "Nigger Jim" in
a review of _Puddn'head Wilson_ from _The Booklover's Magazine_,
February, 1904.  This predates Paine's use of "Nigger Jim" in
his _Biography_.

I still wonder if "Nigger Jim"--that exact phrase--appears in
any of Twain's works.  Oh, for _Huckleberry Finn_ on CD-ROM, so
I could have my computer look for it in a second...

Robert Dagnall