We may need to add another number to the mix, the Nokia Lumia 922. A series of tweeted rumor is out there by @NokiaPureView who is claiming to have handled a Verizon Lumia 9XX (presumably the 922). The tweets mention that the mysterious device has a "matte finish, Pureview, is thinner, lighter and better."
Granted no pictures were offered on this mysterious device and according to @NokiaPureview Nokia CEO Stephen Elop doesn't want any pictures taken of the Verizon Windows Phone.
Take it for what it's worth. While a tweet isn't the most reliable source, they have proven to be right before. For now, we'll ride the fence and say that a Verizon Lumia 922 isn't beyond reach in the future seeing how the 8XX series is being handled. We just don't know the extent of AT&T exclusivity on the Lumia 920 and it may prevent variations from hitting the shelves right away.
One thing is for certain, we all may need cheat sheets to keep up with all the Lumia model numbering that is being generated.