Here's a review of a (slightly farther along) prototype Ngage. You can clearly see that the IMEI sticker of this proto matches the look of Buyatari's, and is different from the demo "proto" units and retail models. You can also see that it has a different color casing (different still from the Green). Since this one won't boot up, those interested can check out the link to see what the system screens probably look like (different version but probably pretty close.)
You said they were not uncommon which is basiclly saying they are common. It might not qualify as one-of-a-kind holy grail status in your eyes but I'd say it still qualifies as rare.
Taucias is right, in the UK there are quite a few of these floating around the open market, if anything it's harder to find a demo unit at times. But you are right, it doesn't mean it's not a rare item.
Well, for one thing, the army green back plate isn't rare at all. They're common as muck here. Is the front plate green as well on the demo units? The store bought ones that have the green back plate, have a silver front (like this).
I also bought one of these on EBay, it was cheaper than a normal one, because the phone part isn't functional. It plays games fine though.