Not only did he play in Muddy's band on those last 3 records, he produced them as well.
In about 1977 in chicago, word filtered down via some club management that Johnny was going to be in town sitting in with the Son Seals Blues Band. Word was he was in great physical shape, clean and sober and ready to rip it up. I worked with the guy who gave me the info. This guy was the program director at THE quirky/ecelectic FM station in town, and part of what the station did was to send Ken out on Saturday night to record somebody noteworthy live, then mix it down and broadcast it on Sunday night. I talked Ken into letting me tag along.
Now, Son Seals was no slouch of a bluesman. You have to really be something to get the word "mighty" used in reference to you in Chicago, and Son was one of those guys who had it attached to him all the time.
Nevertheless, at one point in the evening, they were taking a whack at Stormy Monday. Son held forth with a couple of streaming choruses that I'd give my right nut to be able to play, and then it was Johnny's turn. He killed the entire room with one note. The recipe was this: Wait until the second bar while the band backs off some. Turn your volume off, bend up to just the right note, hit it, and then roll up the volume while un-bending the note. It was the perfect note. No kidding, half the band staggered around and the audience went apeshit.
Some of the stuff from that night made it on to a Son Seals album, but not that moment. That's when I knew that he really was the shiznit.
I'd brought a few LP's with me in hopes of an autograph. Now, Johnny's almost completely blind beyond about a foot away, and he had my decade-old copy of The Progressive Blues Experiment practically up to his nose so he could see what he was autographing. When he saw what it, he remarked, "Damn, I didn't think anybody bought this record".
He's elderly and fragile these days, but he's still the real deal.