Walt, It's an unofficial class for now, but basically it a Narrow Class Chassis with wide rears and MX-5 body. That's at least what most of us have been running. A few of us just switch the rear wheels to run in both the Narrow Class and the MX-5 class with the same chassis and body, and there is no setup changes needed to be made between the switch. I can push it harder with the wide rear wheels, and I'm on 2 wheels in the fast corners much less!
Hopefully Traveler2 can make it official with some set rules soon, as it seems like there's enough interest. It's more of a true "stock" class, since it's a currently available body, narrow only front end, and only so wide of wheels can fit.
Anyway, hope this helps some! Drew