I'll have another look with a set of brand-new eyes, thanks
I'm guess that the regular 8-bar blues isn't just a 12-bar blues without the first 4 measures, normally?
I had already scanned through the wikipedia article but that didn't help me much for the problem at hand. I like the song, and have a nice interpretation of it by Eric Bibb and Jean-Jacques Milteau on a disc called something like Leadbelly's Gold (heard them live last summer with that programme).