I've just spent a lot of time listenint to the "That's called that ..." section of "A Rag", trying it at different speeds and with different EQs, and I'm 90% convinced that the full phrase is "That's called that sharp ball." I'd had an idea that it might be "sharp balling", with the "i" sort of swallowed and the "g" dropped so it was pronounced "sharp balln", but I just can't find an "n" on the end of the phrase.
Then it occurred to me, in a bit of a "D'oh!" moment, that "A Rag" is on the Revenant Charley Patton boxed set, so its lyrics are transcribed there. Dick Spottswood and crew agree with "I'm the only man can play this here rag, call it "A Rag"", but unfortunately give the line currently in question as "That's called that "shocko".", with "shocko" presumably carrying the definition: "Darned if we can tell, but it sounds like this". Not a lot of help!
So unless someone convinces me otherwise, I'll live out the rest of my days hearing "That's called that sharp ball".
