The slabside is the AC Ace body of that period. It was what was originally fitted with the 260 then the 289. As i2f states, it looks very much the English roadster.
The FIA has flared front fenders, wide "hips" in the rear (just 1/2" narrower than the 427 rear), cut back doors, a different hood scoop than the 427, a front-braced roll bar, and it retains the original front opening of the slabside (a snarl, instead of the 427's gapping mouth) with an
oil cooler scoop below. Think English roadster on steroids.
I set out to buy a 427, but went completely nuts over the body design of the original 289FIA, thus my purchase of an ERA. Love it! It's an expensive product, but I havent been disappointed in the slightest.
DD