Apparently the part comes from a Chinese supplier who, according to iFixyouri, is reliable. But not even they know if it is for an actual product or a prototype. This more or less lines up with past rumours of a metal backed iPhone and a smaller dock connector, but then again, you could probably find rumours to back any combination of features you'd ever think of.
Also worth noting is that the sides of the phone are part of the back plate, which would essentially create a box for the screen and guts to sit in. And not to jump the gun on this, but if that subtle two-tone look ended up in the final product, we wouldn't be opposed. [9to5mac]