The letter "I"?
I suppose there's some sort of incremental improvement over IV, or XIOM wouldn't have come out with V. I haven't even tried to make sense of the entire line yet.. no idea what the differences are between the Vegas and Omegas, etc. etc. For that matter, I've no idea what the difference is between Roxxon and Rasant, either..
(Or maybe they WOULD come out with "V" even without an improvement - my theory about why most rubber companies eventually end up with encyclopedia-like catalogs is because if they don't come up with 2-3 new rubbers every year they'd lose sales and market share.. and they can't just kill off old ones because there'd be this loud gnashing of teeth on the TT forums from people who'd claim one of last year's rubbers was the ultimate and this year's version is garbage...)
Iskandar
But a letter on our keyboards none-the-less.