Original Mono Mixes
In the 1960s mono was “king” as far as pop/rock music was concerned. Stereo was mostly the interest of “audiophiles” who were mostly listening to jazz and classical recordings. Because of this situation (and because it was what they grew up with themselves) The Beatles spent almost all of their time & energy on the mono mixes of their albums up through about 1968 when stereo began to overtake mono as the principle way that pop/rock fans listened to music.
When The Beatles albums were first put on CD in 1987, they were mostly released in stereo (although the first four albums were released in mono.) This means that the mono mixes of albums like Revolver, Rubber Soul, & Sgt. Pepper have been extremely difficult for Beatles fans to hear during the CD era.
The Beatles In Mono
Finally with The Beatles In Mono Box Set (released on 9/9/9 on the same day that The Beatles In Stereo Box Set was released) the original mono mixes are available on CD.
This box set includes every single original mono mix The Beatles released in the 1960s and even four mono mixes that were never before released (“Hey Bulldog,” “It’s All Too Much,” “Only a Northern Song,” & “All Together Now” on the Mono Masters double disc non-album tracks compilation.)
It does not include Abbey Road, Let It Be, or Yellow Submarine (although as you can see above, the actual Beatles songs on the album are included as a part of the Mono Masters set.) Why not? Because those three albums were never mixed in mono in the first place!