His looks different, perhaps its just a non functioning cosmetic part of his lower. If you need a new mechbox pin there are a few companies out there that make replacements, King Arms for example.

Also I spy with my a little eye a PGC 7mm prowin mechbox in that set up.

No it isn't. His is a GENII while the one from Huang is a GENI but it's basically the same. I just asked him. He said it's the real steel. I suspect he modified it to fit though. And to answer your question bleaches, it's to prevent wear on the trigger of the Ar. In short it prevents it from breaking i guess.

ah gotcha. learned something new