Think of NX72 as the series and the last digit as indicating very minor difference.
The last digit is also used for models from different countries
Examples of model differences where the last digit differed, from other series are:
models with/without a satelite tuner next to the digital and analogue,
models with a metalic or a black foot,
models for 50/60 Hz power supply (Europe/USA),
models with the wifi-dongle optional or with wifi build-in.
I think NX72 does not exist as such, but is just short for the model in various countries.
