Thanks everyone for your comments (I knew I'd get some good feedback here - it's a great forum and so glad I came across it). Re posting in this thread; didn't want to break any rules but did ask first! Thanks too for the links and will definitely follow them up. As for getting my money back - I bought this coin in amongst a bunch of others, most of which I am sure are genuine and probably worth more than what I paid for the lot. They were from an auction of someone's collection who had passed. They probably didn't know themselves (or maybe did and just kept it for some reason). I will probably start a new thread on my collection (if that is the way to do things - still learning so any advice welcome).
I would speculate that the “1873-S Dollar” is a fairly recent Chinese fake. The Chinese use that “shoe polish” toning technique to simulate age. They end up with dark areas in the devices and bright white fields.
Thanks everyone for their responses. Brian did respond to my email saying that he was "fairly certain the s was added to an 1873 Philadelphia minted Seated dollar." This was based on my photos but a definitive opinion would require him seeing the coin up close. I will keep it as it could be useful when comparing to any new ones I get in future.