S1 has the ignitor on the rear of the engine S2 does not.
you should be able to determine what you have by the plugs anyway.
also there is a S3 version of it as well. mechanically not a lot changes - electrically though its different,
slippy,
whilst I agree with you I too reference them by 1,1.5,2,3 as it is easier to talk to the customer with those numbers, when talking to nissan though I reference by chassis number and model. its easier when dealing with O/seas suppliers to give correct info than to half ass it.