The two-door Honda Z was a kei car/city car designed and built by Honda sold from 1970 to 1974. US sales (only with the larger 600 cc engine) ended in 1972, well before the 1973 introduction of the Civic. Marketed as the "sport coupé" version of the N600 "sedan", the two models were sold side by side at American motorcycle dealerships until the first stand-alone dealers opened with the Civic on their lots. Total production was 40,586 units. The car was first introduced in October 1970.
The export name of this car, the Z600, simply reflected the engine's size of 598 cc. The smaller engined Honda Z360 was available in Japan (and other markets, such as Australia) with a 354 cc twin. The "Z" designation was resurrected in 1998 and was discontinued again in 2003.
Source: Wikipedia, 2011