Here's a good read about the song. Note that the song used a basic three chord structure, but some of the people who do the song will add what one might call 'solo' or lead lines--stuff a second guitar might add to the basic chords. They often accomplish that through the use of nonstandard tunings. (Nigel's post explains that in Memphis Minnie's case.)

https://books.google.ca/books?id=-w-uGwm_LhcC&pg=PA270&lpg=PA270&dq=new+dirty+dozen,+blues+song&source=bl&ots=tUka3KHC37&sig=ACf