Dear Internet,
I am pretty sure everyone has it wrong regarding prototype or test unit.
Before the iOS is loaded on every iPhone that is manufactured, it has to pass a Quality control check, part of which are functional/stress tests.
It is almost certain there is a test software package which is loaded on every iPhone once the hardware is assembled, then overwritten by the iOS once the tests are passed successfully.
In order to cope and identify source of problems, such software would include diagnose tools, as it would be really stupid and not time-efficient to load a new diagnosis software on the phone if a test fails.
Apparently, this iPhone did not had its test-software replaced by iOS, so it can be legit, but in no way it's a prototype.
Of course, the only thing which does not cope with the above is the lack of markings on the back cover, but this might be explained if the markings are applied after the iPhone passed all tests successfully. Why would it be like that is anyone's guess. One possible explanation is that the backs are manufactured just with the iPhone logo, maybe by some sub-contractor, then the text is added according to current FCC rules/requirements, which might change from the moment the product is launched so it would not make sense to order 20 million iPhone backs with an already-defined text. You know it will always be written iPhone on the back - so you order them with "iPhone" text, but the small letters and FCC norms are added later, before the product is released.
I suspect this one failed the tests - see the red sticker around the lightning port which is there to protect the phone from scratches (beware lightning dongle!) while the software is loaded in the phone.
Why it is red? because red = danger/defective, and this sticker should probably be removed after final software is loaded and all is OK. If the phone has it, it's not ready yet - I see this as a visual help for the workers, to easily identify the finished products.
It might look a bit fantastic, but it's just a Quality Management process and from my point of view all falls into place.
PS
Excuse my crappy rusty english - obviously, not my native language ;-)
PS2
Just checked my iPhone 5s - all the small text is laser printed, while the iPhone logo and the apple were made using a different process. It is very probable the laser engraving takes place right before the product is released, as it also contains the phone's IMEI so it's a custom text for every phone.