So I recently came into possession of a bunch of "Tan Kanoka" or more precisely, tan throwing discs. Upon close inspection, I couldn't help but notice that two of the discs, both branded 01-1, had an identical rough texture in the middle. Here is one such disc as compared to a 01-2:
My assumption is that at least one out of every eight discs had such a texture and hence an entire batch of these discs was discarded because they were considered defective. And they'd probably been sitting in a store room somewhere until someone in 2005 decided they might as well give them away at LEGOLAND Billund.
The same logic may very well apply to the black throwing disc.