Anthony,
While I can not validate the authenticity of the signature, I will say that if it is the real thing, that would be one nice piece of music history to have.

As for proving it's authenticity (as well as getting back on topic) has anyone thought to check old arrest records and such? Leadbelly spent a lot of time in and out of jail, so surely there is some item with his signature/real full name somewhere. They usually put your full birth name on legal documents, so perhaps an arrest record, prison inmate inventory, or some such may be a place to look into finding his real name, and a visit with the public records office of the places where he had been arrested may another altenative. The real question is who cares enough to do the legwork?