From what I can recall, Duraspark I and II came in '77 and later models. Before then it was just 'standard' electronic ignition.
The components from 75 on all looked similar. The original plug wires could have said "Duraspark I" or II on them. My '78 had "Duraspark II" on each wire.
DSI was able to adjust the dwell automatically and DS2 was the same as I but with a higher current coil.