As I said in my first reply. The verification you need is the VIN number stamped into the pedal box, not on a plastic tag BSN attached with pop rivets.
The BSN that Mike Drew refers - and the 3000 etc plate you wrote of - is stamped onto a plastic tag that is pop riveted to the car.
it is something that you would WANT to see on a genuine GTS, but like the clock, the emblems and the flares, it can be added to a regular car at a later date.
Larry