They use something called the "emissions publication". It's a listing of every car sold in the UK that should have been fitted with a cat - regardless of whether it was built before the cut-off date. When you look at the MOT inspection manual flowchart for emissions and it talks about an "exact match", meaning that the vehicle make and model has to be an exact match with the listing in the emissions publication otherwise it just goes on age. I imagine that these days, they've just got it on the VOSA database now, so as soon as they put in the registration or VIN, it will search the database automatically and tell the tester whether it's supposed to have a cat or not. Was the Supra an import? If so, it probably won't be listed, so you needn't worry about fitting the cat if that's the case.