Happy 425 day!

the number on the air filter top indicates the torque of the motor, not the hp or cu in

The 425 cu in was the largest-displacement version of the Nailhead. It began as an option in 1963 for the Riviera and was later available on the Wildcat and Electra models. The 1964 and 1966 Riviera used the 425 engine as standard equipment.

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.

https://en.wikipedia.org/wiki/Buick_V8_engine#425