"X" or "TA" heads???

Jadaharabi said
Not an X head. It's a 2.02, 915 U head.

Nice heads by the way.

Damn, this is a bit confusing. So what is the difference between the "X" and "U" (or "O" & "Z" as stated in one of the above posts) heads? Does one flow better than the other? Different springs? :BangHead: