It helps to read the Salearea Guides more than once. As you said, Coach's serial number and coding system is complicated and they made major changes frequently. Some numbers have codes as part of the serial and some don't. Anything made before 1994 won't have any codes in the serial numbers at all. A single reading of the Guides can leave you feeling more confused than before. I re-read them every few months just to keep my memory fresh, and I researched and wrote the darned things.
Also, please don't be confused by other so-called "guides" on the internet claiming to have that kind of information, most of them are just plain wrong. There's a mess of a guide that still shows up on many websites that's been wrong since it was posted more than 15 years ago and the same bad information keeps getting Copy-and-Pasted and posted and re-posted and confusing the heck out of people. Some of its dates are as much as ten years off. The SalereaGuides were researched by me from the ground up and have been frequently updated, the last time was in 2018 when I posted them here. They DON'T cover the periods before 1980, or the total MESS that Coach made of the serial number system starting around 2014 and continuing to the present day, but they're accurate for anything in between.
As others have suggested, have the item authenticated FIRST. If it's a fake, the serial number codes could be wrong and not make any sense. And please keep in mind that a serial number by itself, even if it looks "correct", can NEVER prove a Coach is authentic.