340 TA with x heads ? or not

Didn't the j heads have hardened valve seats, as the x I understand did not.
I've had both, no noticeable difference. The chambers might have been a few cc smaller on the x.
It was nice sticking a 2.02 in a j head, as the valve sat up nice and high, unshrouding the valve for more flow than a sunken valve in a x head.