It seems these bricks were made in the late 1970s without Lego’s authorization, by rogue employees developing new types of plastic and experimenting with their molds and various plastic colors.
It is unknown just how many of these bricks were made, but they are extremely valuable, and in my opinion and many others, very beautiful.
I’ve spent time on the high end MTG Facebook page. I’ve also glanced at the price tags on wh40k starter sets. Those numbers mean nothing to hardcore hobbyists.