[Kevin]

I had watched this video multiple times as it is one of my favorite performances.
I too often wondered what he was doing there with his fingers.

Really scrutinizing it after Lowboy has brought it to my attention, I’m going to step out on a limb here and say he’s not doing anything.

It looks to me like he’s pressing on the wooden comb and not blocking certain notes. Maybe just some right hand support where he didn’t want to block the sound? I really notice when I listen to the audio without seeing the video I can’t pick out where he’s bringing his fingers around.

Williams was known to be quite a character. It could be just some particular tic he used or maybe messing with the other players.
I once heard a well known electric jazz guitarist tell how his pickups were straight wired to the output jack but that he would often tinker with the knobs and switches on his guitar during a song to baffle the guitarists in the audience.

Interestingly in this alternative performance Williamson hardly brings his right hand to the instrument…until he decides to play through his nose.
As I said known as quite a character! Dig that two-tone suit!