I'm sorry too. Unfortunately the early Ergos have been faked often, just like other popular classic and vintage bags.
This is one reason why we always tell posters that just because a style number matches a known style or photos or numbers in Coach's records doesn't prove the item is genuine. The entire serial number has to be exactly right for the style and the known years and plants or there's a good chance the bag is fake. Once in a while we'll see a bag with something off in the number that might be able to be explained by someone at the plant just making a mistake, or as we like to say, "not reading the memo", but 95 percent of the time when the serial number has something wrong, that's a big red flag.
The problem with your situation is that unless someone's familiar with when that style number was made, they'l never realize there's a problem. That's the kind of thing Coach employees (even long-time and very intelligent ones) or even someone with a general knowledge of Coach might not be able to put together. And someone who doesn't know the production history of the Ergos or doesn't have a catalog showing just when that style number was introduced wouldn't see the problem either.
That's one reason why things like online "authenticity guides" are so completely useless. There's so much more to determining authenticity than just looking at a few "Rules" and seeing if the item matches the Rules.
One thing about Coach - there are almost NO Rules - and what few there are almost all have exceptions.
I hope you get your money back. If we can give you any more information to help with your claim, just ask.