Check the tag to see the date of manufacture. Although May 1974 is kind of late for a leftover, it's not unheard of. An easy alternative -- easier, now that I think about it -- is to check the VIN. If the VIN doesn't start with a 133, it isn't a 1973 model year Super and, therefore, can't be a GSR.
Under the front hood (bonnet), there should be a sticker with a color code. It should read "saturn gelb" which means Saturn yellow, and it may be all one word. I don't remember. The color code (L13M) should be listed also. I'm not exactly sure where that sticker is on a 1303.
I am guessing by your use of KPH and "colour" you may be from Canada. GSR's were never imported to North America officially, but that doesn't mean someone didn't bring one over after buying it in Germany. North America had a special edition which was sort of a cousin to the GSR. It was called the Sports Bug. Most were yellow but some were blue. Many of the features were shared by the two special editions -- the seats, some of the black-out trim, the steering wheel, the bumpers, etc. Some of the Sports Bug features were dealer add-ons (supposedly), and they weren't always added on. I don't know if any of this applies to a GSR, but the mirrors prove nothing. As for the rear spoiler, that only proves some previous owner added that on.
Again going with the guess you are from Canada, there was a chap in Maine who owned a GSR clone. I never saw it with the rear spoiler. It was originally a Saturn yellow 1303. He went about his business to collect the right seats, steering wheel, etc., to make it look as authentic as possible. I don't believe he still owns it. It was gorgeous. I would have purchased it from him had I known it was on the market. Maybe there is a slim chance that is the car you now have.
Finally, there used to be a way to write to the VW museum in Germany and get a so-called birth certificate for the car with your VIN. I have never done this but it is my understanding for a nominal fee, they write back and tell you its original date of manufacture, its original color, and some other stuff. I don't know if they will state specifically if it was a GSR or any other kind of special edition if applicable, but certainly color and features would give a rock solid clue at the very least.
Too bad you can't load pics. The GSR looks like the rallye Beetles that used to race, and they remain some of my favorite looking Beetles -- standard and Super.