there are numerous different codes on the ecus... but for the life of me i cant rember what they corespond to.

basically as i remember it....
the S1 ECU has no knock input... (bonus) and has the most ignition mapping points.
very early S2 looms used no knock input as well... however they had fewer mapping points that the S1 ecu.

later and very late 90spec cars had knock sensor input ecu's (which can detect engine noise as knock and hence retard ign) they also have the fewest number of mapping points.
basically if you can get hold of one, you want a series1 ecu as it will work with all looms. and can be made to give the best ignition curve!