Ebay member “jtmaxo” is selling a prototype version of the Apple iPhone 4, which is a test hardware built before the iPhone 4 went into mass production. “The posting refers to the device as a “prototype,” and notes that the serial number written on it is not valid when entered on Apple’s website.” The device has “DF1692” etched in the bottom right corner. According to This is my next, that unique code is meant to allow Apple to keep track of what prototype hardware belongs to a certain employee.
“We are hearing through the grapevine that this particular unit has hardware similar to Gizmodos prototype last year — not identical to the retail version internally, though we dont know exactly what has changed,” the report said.

Source [AppleInsider]