In 1974 I worked a folk club in Auburn NY where we had John Hammond and Steve goodman perform one night. I interviewed them for the college radio station, ( An example of how to embarrass yourself and do a terrible interview as well) and among the questions asked was, What harp do you play and what do you use for a guitar slide?

The harmonica was a Marine Band diatonic, or a Blues Harp depending on the key, and the slide was a 3/4 Crafysman Spark plug Socket. That was 24 years ago tho' thing tend to change.