From what I have read and been told by knowledgeable individuals, only the first 1,000 (or so) 240Zâ€™s had the 8.0 kilogram (17.6lb) flywheel. The change to the heavier flywheel (the 11 kilogram, or the 24.2 lb one) occurred when Nissan experienced breakage issues with the early crankshafts that had only six counterweights. This resulted in serious harmonic vibrations at engine speeds over 6000 rpm which lead to the failure of dampers and flywheels.
Nissan â€œfixedâ€ the problem by replacing the crankshaft with one that used eight counterweights. They also changed the flywheel to the heavier one and I believe that even the damper was replaced with a re-designed one.
If you have seen a copy of the very early printing of the 240Z parts manual, it does indeed have different part numbers for the associated (the crankshaft, the damper and the flywheel) parts.
The above part changes occurred beginning with engine serial number L24-3606, which would have been around mid-January in 1970.
Later on, during the â€œsecond lifeâ€ of the L24 engine in the Datsun 810 and the later Nissan Maxima, the usage of lighter flywheel was re-introduced.
BTW, the fact that some of the 240Z used the 17.6 lb flywheel has been used by some of the more clever ITS racers to justify using the Y70 flywheel as a performance advantage.