IIRC the flask was required for the USA spec PZEV emissions rating, along
with the bladder in the fuel tank. Other countries got neither device but
some did get rear disk brakes.
The NHW11 had an HCAC system (hydrocarbon absorptive catalyst), the NHW20
has the flask, and the ZVW30 has exhaust heat recovery. All 3 systems
lessen the unburnt fuel emitted on a cold start.