I thought I remember someone looking for a Yamaha C foot...?
There is one on ebay at the following listing. I'm not that experienced at posting a link, so I hope I do it correctly:
http://cgi.ebay.com/C-Foot-Joint-Yamaha-684-Heavy-Wall-Flute-YFL-684-New_W0QQitemZ280100227758QQcategoryZ85874QQrdZ1QQcmdZViewItem

It was me, thank you tibbiecow.

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I checked the ad. It is a brand new heavy wall foot 600 series (my Yamaha has a standard wall, but probably it is not an issue). But for the moment the price is out of my available budget.
What the seller relates (having to wait an year to receive it from Yamaha, is what happened to me when I was buying my YFL6xx. Of course I could not wait a year for an unit with C foot nor was I able to order an additional C foot at $640!. That's why I have a 674...

Ahh...Yamaha...I have a Yamaha flute XD Love it. It's 925, i think......

except i originally thought mine was a 684!

Most likely that 925 represents the silver content (sterling silver is 92.5% silver, often marked as .925 or just 925). If it's a 684, that would make sense.