Paul McCartney attends a Marianne Faithfull session

Timeline

Previous session May 17, 1967 • Recording "You Know My Name (Look Up That Number)"

Article May 18, 1967 • The Beatles will participate in "Our World"

Article May 19, 1967 • Launch party for "Sgt. Pepper's Lonely Hearts Club Band"

Session May 19, 1967 ? • Paul McCartney attends a Marianne Faithfull session

Article May 20, 1967 • "A Day In The Life" is banned by the BBC

Interview May 20, 1967 • The Beatles interview for BBC Light Programme

Next session May 25, 1967 • Recording "It's All Too Much"

About

Paul attending the recording of Marianne Faithfull’s LP “Love in a Mist” at Decca A1 Studio

So that would have been in the afternoon before going to Brian Epstein’s house for the Pepper press launch (with Kenny Everett and Linda).

It’s indeed a Mick produced and Glyn Johns engineered Marianne Faithfull Session at Decca, featuring regular collaborator Jon Mark on guitar (the blond guy)

Marianne Faithfull was in studio recording some Lennon & McCartney songs and Paul came along to lend his support.

From Paul McCartney Colorized Triptych, Decca Studios, London, 1967 | San Francisco Art Exchange (sfae.com):

So this session could be an unreleased Faithfull cover of With A Little Help From My Friends exactly a week before Sgt Pepper’s was out.
Now was it on the day of the Pepper press launch (19th) or the next as the Rolling Stones database suggests, I don’t know.