It's a known fact that The Beatles “Sgt. Pepper's” album was Mono and Stereo but did you know that technically, the original Mono release is considered the bible?
According to Richard Lush, one of the 2nd engineers, the album was intended to be a Mono release only but due to EMI's insistence, near the completion, it had to be mixed as Stereo, also.
This presented a problem for producer George Martin and the engineers who had to go back and remix the album for Stereo. As a result, some effects were left out or not done exactly as intended on the Mono version (i.e. flanging on some of John Lennon's vocals). Some of these effects were overlooked in the remix because they were being rushed to mix the Stereo version.
In all actuality, according to Rush, the Mono and Stereo releases are TWO DIFFERENT ALBUMS and the true album is the Mono version.