That newer print has all audio-options: Optical Stereo (most likely Dolby SR), Dolby Digital, a DTS-Timecode and SDDS-sound.
Older Mono-prints just had one channel, I think from sometime in the 60s it became more common to use Dual Mono. Don’t know the exact reason, but SilverWook could be right that it had something to do with the upcoming popularity of Stereo-tracks and the soundheads used for them.