Four-barrel carburetion was standard on all 425 Nailheads, called "Wildcat 465". For the 1965 Riviera Gran Sport and the 1966 Wildcat GS, the "Super Wildcat" regular production option Y48 included two four-barrel carburetors and the intake manifold, which were delivered in the car's trunk, to be installed by the dealer.
Toward the end of the 1966 model year, in approximately May 1966, Buick offered the Super Wildcat 465 with dual 4BBL Carter AFB carburetors as a factory-installed option. This engine is coded "MZ", while the dealer-installed dual four-barrel setup was coded "MW". Only 179 1966 Riviera GS cars were built with the MZ factory dual 4BBL setup, making it a very rare car.