Yes, I have some records of him playing slide guitar. As usual he is very fluid and fast. Being the perfectionist he was, I suspect he played it on his lap.
The pieces I have heard sound like they were produced with a six-string guitar and a slide as Rick describes.
I like "Packin Trunk Blues" very much and his version of C.C. Rider is superb. I have them on an old cassette Columbia Records/CBS Dac PCT30035. It is just called "Leadbelly"
After Rick gives Spaw's right arm for the chance, I will give the rest of him for my chance.
I guess you know, Rick, that there was some film of him. One is from his wedding with Marthe that was made for a newsreel and the other is some amateur footage that Pete Seeger pasted together. A Vestapol video has them.