There is not much of a difference between the two Z's other than the Fairlady was marketed in Japan only. The name Fairlady was carried over from the series of roadsters produced in the 1960's. The name stuck, so now, all of the Japanese marketed Z's to date carry that name.
But if you are refering to the Datsun 240K, It is a Skyline body (C110/210/211) with a 2400 cc motor (L24) for the overseas market.