You don’t get the full picture.
It has nothing to do with how respectable the auction house is or might be. There are specialties in this Speedmaster world that a watch generalist expert is not necessarily aware about, especially when it comes to 80s, since the main hype has been for the early Speedmasters lately, and there is already a lot to learn there...
A subset of 48xxxxxx is correct for a 1997 watch. As well as another very small subset of 48xxxxxx was used in 1985. 48xxxxxx can be correct from 1985 up to 1997 or maybe even 1998, I don’t know exactly but for sure if you’re only looking for a serial starting with 48 you’re on a perfect way to find a wrong watch.
Unfortunately there is a serials table on internet suggesting that the 48xxxxxx serials are related to 1985 - which might work for any other Omega watch but DOES NOT for Speedmasters. And it’s a known thing.
There are still plenty of people using that table for assessing all their Omega watches - including Speedmasters - and this is where the confusion comes from.
Instead of trying to educate the planet on that tiny matter (I used to signal the mistakes on c24 as well, as you just do - but either used to get no reply or guys replied they are correct and I am a wannabe idiot) I suggest you to use you energy on something else, for example learning on how to differentiate a Moonwatch from the 80s from the Moonwatch from the nineties.
You will end up much less confused and happier each time you see a wrong listing on the web.
Cheers
/A
