I'm sure it's the rarest of the three PCE Duo versions, but it's not worth $500. That's ebay BIN pricing for boxed systems, available all day every day; actual sold prices are more in the $300-400 range, with the higher priced ones being boxed, bundled with games, and/or with new lenses and rgb mods.
I suggest you try getting a regular Duo or Duo-R from local guys like Turbokon or Keith. Functionally there's no substantive difference, and you'll get a good deal on a system you know has been fixed up right. You'll miss out on the RX's six button controller, but you're better off with a Avenue 6 anyway.