Captain Rex's phase 2 helmet (when he finally gets one) appears to use a phase 1 visor style, with it being thick at the nose and tapering towards the ears, while the standard phase 2 helmets start thinner at the nose and expand to make a larger blob around the eye.
Standard phase 1 helmet:
Standard phase 2 helmet:
Rex's Phase 2 helmet:
If you look closely around the nose part of his helmet, it almost looks like there's a line where a phase 2 face-plate was welded to a phase 2 helmet, but even if that's the case, it raises the question of why Rex would do and be allowed to do that. Why did he use such a strange helmet?