Thumb up (George) and back turned on on the back of "Sgt. Pepper."
Thumb up and back turned on "Back in the US/World"
And now, one of my favorites. The clues extend beyond the solo albums into works done with others.
John Lennon produced the album "Pussycats" for Harry Nilsson. The front cover tells us that this is a good place to look for hidden messages. Notice the two blocks on the floor "D" and "S." Notice that in between them is a "R-U-G." Get it?
When we open the gate fold cover of "Pussycats" and look carefully, we see a picture of Harry on a stage with Ringo Starr. Harry is shaking hands with someone in the audience. The person shaking hands with Harry is blocked because another person is holding an album cover out. The album being held up is "Sgt. Pepper's Lonely Hearts Club Band."