When Q is explaining to 007 about his new car, he states that all the windows are bulletproof. Then in the same scene Q opens the console inside the car showing the 'extras' and points to a switch that will raise a bullet shield in the back window. Why need the shield if the glass is bulletproof???
Maybe the back window wasn't bullet-proof so if bond was being chased and shot at, he could shoot straight behind him through the window, then raise the shield.
perhaps when Q was talking about the windows being bulletproof, he meant THE WINDOWS. it's perfectly reasonable to assume that this does not include either the front or rear wind screen. therefore, the rear wind screen would most certainly be in need of additional protection, in the form of a bulletproof plate of some kind.
First, there is NO such thing as bullet-proof glass, only bullet RESISTANT glass. The raisable screen was there to provide additional protection.
Second, in 1964 bullet-resistant glass technology was still relatively new, so additional protection was required. Watch the scene where the front windscreen was machine-gunned; the glass suffers noticable damage; two shots hitting the same spot would almost certainly punch through.