Brian Wilson himself oversaw the re-mixing into stereo. They worked from multi-track tape, and panned each track accordingly.

The original recording was released in mono not because it was all recorded onto one track (or even just two tracks---it was a multi track tape...four track, maybe, bumping from transport to transport?), it was because mono was the preferred format, and many people did not yet have stereos.

For example, although Sgt. Pepper was released in both stereo and mono in 1967, the Beatles took a lot more time mixing the mono version, leaving the stereo version to EMI staff. Mono was still "the" format. By the time Abbey Road came out, it was released only in stereo (if I remember correctly---maybe I'm thinking of the stereo version as "the" version by 1969, yet there still was a mono version...I must investigate...).

Anyway, Brian Wilson re-mixed it to stereo. I much prefer the mono, but it's great to have them both on one disc. Great deal...!!

I understand that the stereo migration was done from the original multitrack. What I do not understand is how one takes a single track (say the lead singer) and migrate it into stereo.
The thing is this;
mono data has all the information in a single channel, regardless of panning (with which you can fool around in mono as well). But stereo data is comprised of two distinct channels, each having data that is DIFFERENT from the other.

How can one take one single channel and from it create a second channel that is very similar to the first, but still different ?
Is it by adding some delay to the second channel ? That in itself does not seem sufficient ...
Also from closely comparing the mono and stereo versions it sounds like much more than delay stand between the two versions.

What is it ?

OJG

That's right, Abbey Road was ONLY issued in stereo.

Pet Sounds was recorded on various machines.
3 track, 4 track, and 8 track machines. Usually backing tracks on one sinngle or 3 track. bouncing over to 8 or 4 track to do
vocals, shackers, etc. Generally speaking of course.
Brian did a huge portion of the vocals himself before the Beach Boys returned from their touring. Redoing them again to include the band.
Caroline No was recorded on 3 track, Brian double tracked the
2nd vocal while mixing at the same time to mono. Hard to do a
remix on that one huh?

Mark Linett did the remix and most of the
Beach Boy remix/reissues, and wonderfully I might add.
There's alot of interviews of him describing his process.
Read "Endless Summer Quarterly"

The fact that Brian Wilson was involved in the stereo re-mixing is also unusual as he is deaf in one ear.

Unusual but true.

On a lot of these reissues, they take all of the reels that were used and lock them all back up. Like on the beatles yellow submarine DVD, they took the string work reels, the vocal work reels, the band reels, these were all recorded on diifferent tapes and used to bounce onto the master tape. by going back to the work reels, they can grab the tracks pre bounce.

OJG wrote: I understand that the stereo migration was done from the original multitrack. What I do not understand is how one takes a single track (say the lead singer) and migrate it into stereo.
The thing is this;
mono data has all the information in a single channel, regardless of panning (with which you can fool around in mono as well). But stereo data is comprised of two distinct channels, each having data that is DIFFERENT from the other.

How can one take one single channel and from it create a second channel that is very similar to the first, but still different ?
Is it by adding some delay to the second channel ? That in itself does not seem sufficient ...
Also from closely comparing the mono and stereo versions it sounds like much more than delay stand between the two versions.

What is it ?
OJG

All the different mono tracks from the multitrack masters can be panned around wherever when remixing to stereo format. Reverbs can be done in stereo, perhaps panned different from the dry source. Drums can be panned around to simulate how they might sound in person. One backup vocal can be sent to mostly the right channel and another can be sent mostly to the left channel. BUT a vocal can be panned straight down the middle and it by itself will sound the same whether it is being recorded onto one mono track or a stereo track (2 tracks). So the two distinct channels don't HAVE to be different from the other.
Hopefully this helps answer your question, but I think you might be overthinking the process.

Thanks for your answers folks.
I thought it was more than just panning that was taking place in a mono to stereo procedure, but perhaps you are correct Reggie in that I am over thinking.

OJG

It's pretty much panning.

True, volume control and EQ and mic placement can change the stereo field from "front" to "back"...

...but left to right stereo is pretty much panning. I mean, unless I'm completely whacked and there's something else I'm not thinking of...!!