Typically when you buy a new filter, the gasket comes with it. At least when you buy from "fillingstation.com", it does.
I've never bought a filter cartridge that didn't come with a new rubber seal. It doesn't matter what brand of filter or what company they're purchased fro, they all come with new seals. I have so many stashed because the old ones often don't need to be replaced that I'll never use them all..
The seals for those filters are normally only about 1/16" thick or maybe a little more. Don't worry about the 3/32". That's correct. Your old one may or may not be correct and likely swelled over the years.
Your filter is definitely not an S-6. The OD of the seal on an S-6 is 5 1/4"
