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.
Injection molding in steel molds is the way to go. It can be done at home, but only by the most serious of hobbyists. (Hell, I could probably make an aluminum mold or two, now that I think about it. At $500 a pop, it could be worth the time…)
The lego tolerances are so tight you would not get it accurate by a hobbyist, many mould makers have struggled with how tight the tolerances have to be. You even have to account for ambient tenoerature when cutting the steel in case your nold has expanded from cutting heat and environment temperature as well as typical cutter wear and deflection, and then there is the polishing aspect