You may already know who Alan Blumlein was. In case you don't, the short version is he's regarded as the "father of stereo", having obtained a patent for the concept in 1931. Unfortunately, he did not live to see stereo recordings become mainstream. 
 
Here you can hear a few of Blumlein's original test recordings from the early 30's, some of the earliest stereo recordings ever made. (Note that some selections will not be available to North American users. It's a BBC thing.)
 
I'd like to know how these were re-recorded, as the method used by Blumlein is not compatible with modern record players. But somehow, the audio was extracted from the original 78-rpm disks, thanks to the efforts of the late audio historian Roger Beardsley.

Great site bit! Thanks for the post.

Most of there were either MS or the "Blumlein" array as figure-8 mics were the most useful to the task of stereo recording.  

My understanding is that Blumlein's experiments predate M/S encoding by at least 20 years. It's true that he did use a coincident pair for recording, but I was referring to how the data was physically stamped onto the disks.
 
Blumlein's patent called for side-by-side tracks at an angle to one another, as opposed to the side/bottom method that was adopted in the 50's. Blumlein's method never caught on because it wasn't compatible with existing mono records. So if Beardsley was working with original pressings from 1933, he'd have to be very clever to read them properly. I'm wondering if a laser-based stylus could have been adapted.

A French guy came up with a way of visualizing (audible) vibrations by etching them in soot on a cylinder.
 
http://en.wikipedia.org/wiki/%C3%89douard-L%C3%A9on_Scott_de_Martinville
 
About 7 years ago the guys at Lawrence Berkeley National Labs (yes your tax dollars at work) were able to digitize scanned images of the soot making it playable.
 
http://www.nytimes.com/2008/03/27/arts/27soun.html?pagewanted=1&_r=0&hp
 
I guess the current state of the art technology still involves capturing a visual image of what ever they want to digitize using microphotography.
 
http://irene.lbl.gov/
 
So yes it is quite possible they used an optical technique and then digitized from there.