I might be wrong, but I don't think GB500's ever had a factory optional fairing. Which seems wierd since it's just begging for one. The fairing in your photo is an aftermarket item made by the company EGU of Germany. It looks to me like they may have started with and modified a Ducati 750/900SS fairing. The Ducati fairing is quite often used as a starting point for a new design due to it's fairly large size (for a half fairing) and it's solid classic lines.