GE got out of the ATS business some time in the late 1970s - early 1980s when UL began to require separate listing requirements on them, then got back in when they bought Zenith in 1999 (Zenith was already using GE breakers in their ATS). So given that this one is not a Zenith product, that means it pre-dates the internet, which is likely why you can’t find info on it.
If this is already installed and grandfathered in, you can possibly repair it but if it’s an old used unit that someone wants installed, it’s probably not going to be legal since it will not have UL1008 listing (which is why GE abandoned the line)
