The mystery plastic is probably POM. Yamato was one of the first companies to use it in toys and it's a crime they didn't use it in the original. It's plastic that has properties of metal, it's cool to the touch and strong, yet almost feels ceramic. They usually only use it in high-stress joint areas and hinges.

I think this one marks the end of the MZ23 line since I can't see them doing the one from part 3 or going the extra mile to do the villains. Interestingly the factory color release was based on an old illustration from the preproduction of the OVA, so it wasn't just something Yamato pulled out of their ass.