Hi, many thanks for Your help!
I have searced the parts Van Zweeden has for sale. I didn't find anything there. But I have today sent them an e-mail to hear directly from them
The normal 924, and also the Carrera GT has an thin aluminium bellypan. The Rallye cars have a much tougher, thicker and heavyer underbody protection plate to protect the car and engine from big impacts of rocks and such.
Unfortunately my friend and I don't know how this Rallye-underbody-plate looks like... Such a plate was once installed on this GTS, but now it only has the normal thin aluminium plate, I guess it is much like the one on Your normal 924.
My son has a 1984 924, so I can check if that is the same as on my Carrera GT. I would think it is!
Yours, Ole Petter