both 125kHz and 13.56MHz
Chinese providers often type out all features of all products, in one list. Then you have to be careful when ordering to ensure you get the exact product you wanted.

So in this case I suspect you can get either or.

@jonwaller said:

then set the ring's public code to that address, and the ring's private code to the private key of that address.

I think there has been reports of the private side of the ring interfering with the front side. Most likely, some NFC readers can be strong or smart enough to read both the public and the private side.

There are also methods, using special-made antennas and such, to scan NFC chips from a much larger distance. I probably read about it in news reports from some BlackHat or DefCon conference.

Another option is to just store the keys on your ring and use them to interact with your mobile phone so your mobile phone never stores your private key

Sorry, but storing a complete private key on the ring is probably a bad idea. Since the merchant taking payments are surely funneling all payments via some bank or payment processor, they could just send your public ID and let the remote payment processor handle things. They could do an instant debit or even a delayed credit transaction. You could have traditional (and costly) option to dispute the transaction.

With banks testing contactless payments (NFC) for sums under 25EUR, the same could be done with NFC rings, but that would surely mean the ring must use other NFC chips and the whole process would probably get more expensive. The brave could perhaps dare to extract the active part of a Paypass and try to squeeze that into a ring. But it'd surely be illegal for the stores to receive a payment unless you have an unharmed card with your signature etc etc. So, trouble.

hands me the decive to enter my PIN.

Would be great to avoid giving away the PIN to anyone looking. Or if the hardware is cloning. There may have been a bluetooth-based ring up for pre-order (Kickstarter?) with a display, which could solve some of this. But generally just pre-approving myself with a budget for coffee-places, another for lunches, etc would allow payments without PIN in these places.

@Lokki said:

an NFC enabled sticker for the back of your non-NFC phone

Maybe this sticker isn't a classical N203 but instead something with private-public crypto built-in? A decent phone should be able to read and give a good guess at what chip is used in that sticker.

ring-initiated Ripple/Stellar transfers?

Maybe multi-sign solutions can be interesting here? New wallets with this functionality is currently in beta, if I've read the news correctly.

Ideally, someone with shorter shipping time from these merchants could get one in the mail, do some testing, then if it works send it onwards towards someone in Europe at maybe 2x cost. Documentation and installation instructions are typically very weak on Alibaba and similar websites.

These hardware lists really could benefit from some kind of Wiki or table format. I'd like to have a list of: manufacturer, model, price range, test results, warranty length, links for where to buy

I've got a list of candidates (made in China) for testing:
Guangzhou Douwin Electronics Limited [China (Mainland)]
Low price smart card handle nfc door lock / High grade NFC security safe door lock
(FOB Price : US $31 - 35 Min. Order : 100 Sets)

Shenzhen JinBeiEr Intellectual System Limited Company [China (Mainland)]
handle lock nfc door lock with intelligent card
(FOB Price : US $30 - 40 Min. Order : 1 Piece)

excellent guide! One photo (justa bove "Remember to press your “Back” button, or the task won’t be saved!!") is gone -- hopefully it's not important for the guide!