This song starts with a pick up bar, there is an unwritten rule if you use a pick up bar you need to extract it from later in the progression. “Perhaps it is a written rule” A slow blues typically will start from the” five” this is so everyone has a roadmap.
But here he treats the entire introduction as a pick up bar. He will later extract a bar here, two bars there. This is an easy song to mess up; it requires the utmost attention to detail. There is more meat here than a rib eye steak from whole foods market.
LOVE me some Billy Gibbons. Check him out playing LaGrange in a Live at Daryls house segment, available on youtube. in the middle of it they cut to a quick story he tells about discussing string gauge with BB King. Great stuff. Billy's one of the coolest cats ever.