I'd ask over on the Storm forum. You might get a quicker answer there if a Storm rep sees it.
My guess would be Accu-Tread. The Curelyon coverstock was last used on the Barbed Wire, I think, and before that the Hot Rod. By 2001, it was a pretty tame coverstock. **EDIT: Pro-Glide was on the Wire balls.**
Judging from some of the other Storm balls made around that time, PRO-Thane and its derivatives were used a lot, as was Accu-Tread. In fact, those two and a coverstock called "Cadmium" are the only three I could find on any Storm ball made in 2001.
Jess