I'm not convinced they ever made X-clones in X plastic. If they did I never noticed.
If you have one stamped as X plastic, does it actually feel any different from the normal runs in Cyclone plastic? Sometimes someone in the factory just stamps discs without paying attention or knowing what they are doing.
X plastic is just a candy blend. And there are lots of variations in those blends. Really Cyclone plastic is a type of candy blend. It was the first high tech plastic to be molded into drivers.
So without looking at my collection I would guess none of them are stamped X plastic but all of them are a blend, and every run the blend may be a bit different from the others.
I personally liked the flat top ones because they were the most overstable, so I have lots of those.