Hmm that's so weird. A mate of a mate is an ET fanatic and told me that they were all 30's, but I'm not sure if he really knew what. Going off what Nissan do with other engines, being a 1.5turbo etc, it would definately be a 31 series gearbox.
Mighty, no just because its a 30 or 31 series does not mean it will bolt up to the CG engine. That 31 and 30 just represent the model series of the gearbox internals, there is the other 4 digit code to consider which tells you which bell housing pattern it is. I.e. N13 31a boxes do not bolt up to the micra's engine, only N14/N15 31's do because they are also the FB40 bell housing bolt pattern. etc ..