My point was that the color of the rear wall should not be different from its surroundings, because it's not a detachable piece. These photos didn't persuade me, but I've found some modern day ones that show it in silver indeed (and I wonder why).
There are a few more things that look different from what I've seen on other Gr. A BMWs, but they seem to be consistent with the photos of this car that I've found now.
You've probably seen these pages (there's an engine pic on one of them, by the way -- not sure if it's authentic, though):
http://www.bpa.cz/CZ/tisk.html?rubrid=40
http://auto.idnes.cz/bmw-635-csi-har...to_ojetiny_fdv
One thing you may want to keep in mind if you are going to build more 635s: the black wire that runs from under the expansion tank cap towards the firewall on your model is actually not a wire but an overflow hose. The other end is not connected to anything, it just points towards the ground.