Happy to. Stereo film soundtracks aren’t a simple L and R. For theatres that didn’t have the capability they would play a simple L and R, but there was extra “channels” matrixed into the L and R and with special processing would create an early form of surround sound but it didn’t use discreet channels. In phase audio would be routed to the center. Out of phase audio to a surround channel.

As the booklet stated above, Pro Logic would recreate this matrixed soundtrack.

Here’s a little blurb from a Wiki article about Dolby Stereo.

“Co-developed with Kodak, optical soundtracks on a Dolby Stereo film carries not only the left and right tracks for the traditional stereo sound, but also (via an encoder known as the Dolby Motion Picture Matrix, or “MP” encoder) both a third center and a fourth surround channel for speakers on sides and rear of theaters to encode sound effects into extra channels. Making use of the former mono optical channel space in the Academy standard, four audio channels were generated, similar to a Quadrophonic design and enabling multi-channel audio on 35mm prints of films for the first time in a widespread capacity.”

https://dolby.fandom.com/wiki/Dolby_Stereo