It had a plastic "rf window", ala cellular ipads."A all aluminum back would cause antenna issues" Wasn't the very first iPhone all aluminum?
Then they were all plastic back. 3G/3GS
Then all glass back. 4/4S
Now they are back to having aluminum with glass "rf windows". 5/5S
And now coming back to an all plastic back. 5C, 6C?
I see a pattern developing.