On September 27 1967, George Martin had conjured up an excellent orchestral score for John Lennon's composition "I Am The Walrus" in which some horns, strings and clarinets were added as well as having a vocal backing consisting of the Mike Sammes singers who were professional backing singers of the era. All of this was recorded with the orchestra going first and the singers going on second to enhance the song much of the delight to the Beatles, I"m sure !!
Two days later on September 29 1967 John Lennon decided to add a live radio feed from a BBC broadcast onto one of the mixes (mono mix 22) directly to tape. Thus the first of the mono mixes was created by having the first half of the song from mono mix 10 and the second half of the song from mono mix 22. You can hear the edit at around the two minutes mark - the combination of the mixes were now called mix 23. The intro was also trims to four bars and the extra bar of music before the words "Yellow Matter Custard" remains in the mix.
For the stereo mix (which occurred on November 06 1967), the song has true stereo (stereo mix 06) for the first half of the song and second half of the song (stereo mix 07) has bass on the left channel and treble of the right channel due to the fact that the live radio broadcast feed was added during mixing and therefore could not be separated in a different channel. At the end of the song during the fade-out, the signal is panned left and right across the spectrum. This mix also has the four bars during the intro but the extra bar of music before "Yellow Matter Custard" is taken out of the mix.
The two mixes above appear on the North American "Magical Mystery Tour" LP (mono and stereo) and the North American single (mono).
On November 13 1967, somebody at EMI realized that the two mixes were of different lengths and an attempt to correct that saw the extra bar before the words "Yellow Matter Custard" removed from the mono mix and used on the British single.
On November 17 1967, just before the British EP was finally cut, it was decided to use an alternate stereo mix for the first half of the song using the original six bar intro (stereo mix 25) edited onto the second half of stereo mix 07. This was added onto the master reel of the EP created another stereo version of the song.
Finally, the "Anthology" DVD has the first complete true stereo mix of "I Am The Walrus" by spreading out the instruments with the electric piano on the left, percussion in the middle and the guitar on the right), the orchestra is split far left and right and the radio feed is lifted from the original broadcast with added radio static. This mix was also the case used on the Apple double LP/CD "Love".