He still has them made at GM. It's a thicker-edged version of the Mana which he has been using since the beginning of 2015. Since then the shape hasn't changed. Previous to this his bats were a similarly chunkier version of the Six6 (personally I prefer this profile. In fact so much so that I actually own one of his bats from 2014). New Balance retail bats are made by Sareen. Millichamp and Hall as far as I know have absolutely nothing to do with new balance. A cursory inspection of one of their bats would clear this up as they are vastly different in many ways.
Hi welcome to the forum. I have it on good authority that GM don't make Joe Root's bats any more (they gave a lefty over to a friend who has been working with gm for almost 10 years.) And sondhi make the nb bats. That is clear when you see that their shapes are the same as the slazenger bats.