To be more specific, the 500s that raced in NZ had 3-litre engines, the 555 Supersqualos 3.4 (Tipo 750 and 860 respectively)
And yes Bruce, they were pretty fast (first at Wigram, Ryal Bush and Dunedin in 1956 and 1957 and the GP in 1957, second in the GP 1955, 1956, 1957 etc)