Finally some participation in this research!
Of course it is hard to tell what is what and which is which, that car definitely doesn't look like rally prototype of the articles. ^^
I have gotten not much help from Opel Motorsport since they say they do not have any documents pertaining to the 4WD prototype. They did manage to find someone who worked there at the time this car was built but he doesn't remember much except that the only prototype he knows about was a "narrow body" all white Manta.
However, the rumors that "more than one" prototype was built might be getting more serious since you can see in one article that the car lifted up to photograph the undercarriage is indeed an all white narrow body Manta and appears to be RHD! The drivetrain schematic is also in RHD. Then there's the actual rally prototype shots when testing on the tarmac and "wet grass"... it is clearly LHD and sports all the other exterior mods as noted before. This makes it more likely that at least 2 were built (one on a narrow road car, and another in rally spec). Albeit the former could have been converted to the latter but to go as far as switch it from RHD to LHD too? Unlikely in my opinion.
Then there's the whole bickering in the unlikeliest of places... a slot car forum!http://slotadictos.mforos.com/21954/11259511-novedades-avant-slot-2014/?pag=2
However, it certainly gives some leads to follow as to where these cars might have ended up. I have sent a few emails. Hopefully, that part of the story will help figure out how many there was...