Apple's Watch is designed to be both fashionable and functional.
Photos of 'Plastic Low-Cost iPhone' Likely an iPhone 5 in a Case
Our first thought upon seeing the photos was that it is likely simply an iPhone 5 in a plastic case. A lack of an Apple logo or discernible text on the back of the device is unlike many Apple prototypes, and the Lightning cable looked to us to be plugging into a recessed case rather than a flush port on a device.
French site NowhereElse.fr apparently had similar thoughts [Google translation] and performed some analysis of one of the images. Adjusting exposure and contrast on the photo better reveals the bottom of the device, and it certainly does appear that the Lightning cable is plugged into a hole in a case rather directly into a device. Other features along the bottom of the device are also consistent with the iPhone 5: speaker, microphone, and headphone jack.
As a result, we believe these photos are almost certainly an iPhone 5 in a nondescript plastic case rather than an actual low-cost plastic iPhone or a rear shell from such a device.