For what it's worth, that particular distributor (two oval connectors) is referred to as the Delco 'HEI-EST'. The coil and trigger module operate in a stand-alone state with just power and coil... about the closest you can get in simplicity to a breaker-point distributor, but with the added feature that it requires no mechanical spark advance mechanism, the module does it all. There's several different modules available, which have slightly different ignition curves, and there's 4, 6, and 8 cylinder caps and reluctors available. For an odd-fire V6, one can use an 8cyl reluctor and cut two wings off.
They were all set up to work with the 'square brick' type ignition coil, and they threw a scary-nasty snappy-hot spark.
From the factory, they're naturally explosion-proof, so they're ready for the marine environment with no modification... as a result, they were very popular for converting GM Marine 4, 6, and V8's to electronic.
The secondary connector (4 wire) is used where computer 'adjustment' of the spark was done. With a connector attached, the distributor would respond to computer suggestion and knock sensor(s) to alter the default advance curve, but with the 4 wire disconnected, the distributor would follow it's internally based default advance curve.
Great little distributor, and it did not suffer from the earlier (coil-in-cap) advance mechanism or spark scatter issues. Bushings lasted really well, and once sealed, they'd stay clean inside just so long as you didn't drive the engine in short-distance cycles and wind up with a plugged up PCV system. When the engine doesn't get fully warmed up, condensation in the crankcase accumulates, and it gets everywhere (not just in the distributor). Corrosion inside that distributor is basically always from the engine spending too much time in a 'cold running' state, and not enough 'warm running'.
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