The Beatles’ 1969 album Yellow Submarine is viewed—by critics, fans, and the band itself—as its least significant, weakest release. The B-side was made up of producer George Martin’s score for the film of the same name and as such was of little interest over the long term.

Of the six songs on the A-side, the charming but inconsequential title track had appeared on Revolver years earlier and “All You Need Is Love” had been released as a single and placed on the U.S. version of Magical Mystery Tour—plus it’s so vapid that once you hear it once you never need to hear it again.

But in between the two previously released songs comes a brilliant four song burst that wasn’t available anywhere else until The Beatles In Mono box set came out 40 years later. “All Together Now” is a characteristically whimsical sing-along by Paul McCartney, while Lennon’s “Hey Bulldog” boasts one of the best riffs he ever came up with.

Both songs were recorded specifically for the film. “Only A Northern Song” and “It’s All Too Much” were both Sgt. Pepper-era outtakes by George Harrison; the former a dark yet detached critique of the exploitive nature of music publishing, the latter a startling rock jam that drones on for over six minutes, the four Beatles augmented by four trumpets and a bass clarinet.