Pretty smple really
The paint code is Y229
http://www.falcongtclub.org/PaintCodesXAXB.asp
That is CUB brown
If the VIN number on compliance plate, passenger shock tower and block starts with JG66 then it is more than likely the only CUB brown XB GT coupe
Yes, it is very rare but the colour isnt one of the popular ones
Please note, you would want it verified by someone in the know to ensure it isnt a rebody though.
I know you dont want to call anyone but pm barra64 (the GT historian) and he can help you out.
Good luck with it