The way I've always played it (and heard it) basically goes back and
forth between a barred E chord (A form) with index finger on the 7th
fret, as you say, and barring the D, G, and B strings using my index
finger on the 9th fret to form the B, E, and G# notes and using my ring
and pinky fingers to do what you call the "twiddly bits" on the A string
at the 11th and 12th frets.

So I've been playing the signature riff of "Don't Let Me Down" for the
last 30 years in a C shape of E on the fourth fret.

So I've been playing the signature riff of "Don't Let Me Down" for the
last 30 years in a C shape of E on the fourth fret. It's easy to do
and it sounds great if you're playing it solo. But it recently
occurred to me that George might actually be playing it with his index
barred on the E chord at (no spam) 7, doing the twiddly bits above that on 9 and
10. I watched the rooftop movie and that's what it looks like. Is that
right? Thanks!