X-01 was actually made postwar by the Enclave, several decades after the bombs fell. I'm actually not sure there was an X-02 set, or whatever - the better version of it in the older games was called Mk II.
There was Enclave Hellfire Power Armor in the Broken Steel expansion for 3, though - would be nice to see that make a return, maybe it could be largely identical to X-01 armor stats, but with swapped ballistic and energy resistance.