May, 1968
Syd Barrett, ousted from his role as leader of the underground's favorite group, begins work on his first solo album.
John Lennon, disenchanted with his role as leader of the overground's favorite group, begins work on his first solo album ('Unfinished Music No. 1: Two Virgins') with Yoko Ono.
Lennon/Ono collaborations begin to infiltrate the Beatles' sessions, causing friction among band members (note: the nearly unlistenable first side of 'Unfinished Music No. 2: Life With the Lions' would be recorded live in Syd's native Cambridge on 2 March, 1969).
20 June, 1968
Syd Barrett, Studio 3, 7:00pm-10:30pm
"Swan Lee," "Late Night," "Golden Hair"
Beatles, Studios 1, 2, and 3, 7:00pm-3:30am
"Revolution 9"
Identical starting times, and the fact that Lennon eventually took over all three studios, increases the likelihood that he encountered Barrett again on this night.
27 June, 1968
Syd Barrett, Studio 3, 2:30pm-10:00pm (Break 5:30pm-7:00pm)
"Swan Lee," "Late Night," "Golden Hair"
Beatles, Studio 2, 5:00pm-3:45am
"Everybody's Got Something To Hide Except Me and My Monkey"
Syd's 90-minute break provides some intrigue, as do overdubs of a handbell and chocalho (Brazilian shaker) onto the Beatles song. A chocalho or similar shaker is audible as high-end percussion on Barrett's "Lanky (Part 1)," made the previous month.
After his session of 20 July 1968, the nature of Barrett's musical activities over the next 8 months is not known.
14 August, 1968
Beatles, Studio 2, 7:00pm-4:30am
"Yer Blues," "What's The New Mary Jane"
Four takes of the second title were taped, one a breakdown, the others timed at 2.35, 3.45, and 6.45. After acknowledging John and George as the only Beatles performing, Mark Lewisohn writes: "But Yoko and Mal Evans too--judging by the original session tape--were also joining in the fun." The "judging by the original session tape" comment seems an admission that, although Yoko is identifiable vocally, the names of other participants are not known. Lewisohn further observes:
"...at the end of takes two and four someone vigorously shook a handbell and someone else hammered away at a xylophone. During take four someone deliberately rustled paper into a microphone..."
Without overstating the case, the same activities do appear on Barrett records. Vigorously shaken handbells are heard on "Flaming" and during the coda of "Scream Thy Last Scream." Much vibraphone hammering resonates in Syd's "Lanky (Part 1)," and rustled paper endears itself to fans of "She Took A Long Cold Look" and "Dolly Rocker" (note: I hear no xylophone on take four of "What's the New Mary Jane" on 'Anthology 3,' though a sampling of vibraphone may be evident at 3.13-3.23).
25 July, 1969
Beatles, Studio 2, 2:30pm-2:30am
"Sun King," "Mean Mr. Mustard," "Come Together," "Polythene Pam," "She Came In Through the Bathroom Window"
26 July, 1969
Syd Barrett, Studio 2, 7:00am-12:30pm
"She Took A Long Cold Look," "Long Gone," "Wouldn't You Miss Me," "Feel," "If It's In You"
Barrett arrives at Studio 2 for a morning session--to be produced by his ex-bandmates David Gilmour and Roger Waters. In the wake of a marathon Beatles date, he hurriedly completes five titles unaccompanied, and thus ends the recording of his solo masterpiece 'The Madcap Laughs.'
Conclusion
During the period of 1966-1969, Beatles records featured uncredited guest appearances by Brian Jones ("You Know My Name"), Eric Clapton ("While My Guitar Gently Weeps"), Tiny Tim ("The Beatles 1968 Christmas Record"), numerous unidentified Indian and classical musicians, and others.
If Syd Barrett took part in any Beatles recording (issued or not), it would likely be either "Revolution 9," "Everybody's Got Something To Hide Except Me and My Monkey" (he was actually "in house" and available when these took place), or "What's the New Mary Jane," a free-for-all with some of his musical motifs evident. The first and last titles are Lennon, Harrison, and Ono performances. Although The Beatles A to Z states that Harrison also participated in John and Yoko's 1969 Cambridge concert, I've found no other source that confirms this.
Reference Materials:
Allan, Jon II (editor). 'Late Night' Magazine, Number 3. Barrington, New Hampshire, 1999.
Fitch, Vernon. The Pink Floyd Encyclopedia (First Edition). Collector's Guide Publishing, Inc., Burlington, Ontario, 1998.
Friede, Goldie et al. The Beatles A to Z. Methuen, New York, 1980.
Hodges, Rick, and Priston, Ian. Embryo: A Pink Floyd Chronology. Cherry Red Books, London, 1998.
Kelly, John, and Parker, David (editors). Chapter 24 - Issue 3. London, 1995.
Lewisohn, Mark. The Beatles: Recording Sessions. Harmony Books, New York, 1988.
Palacios, Julian. Lost In the Woods. Boxtree, London, 1998.
Parker, David. Random Precision. Cherry Red Books Ltd., London, 2001.
Schaffner, Nicholas. Saucerful of Secrets: The Pink Floyd Odyssey. Harmony Books, New York, 1991.
http://www.neptunepinkfloyd.co.uk/magazine/2006/09/syd-barrett-and-the-beatles.php