This green circled wire as you can see in the picture below connects to the lower part of the distributor body and it looks like its on top of the wiring for the condenser as well. Is this some sort of grounding wire for the condenser?
It look like it will be the wire that triggers the HT coils.
Inside the distributor there will probably be a modern replacement of the old fashioned points in the distributor. The fact that there is a condenser on the distributor makes me think that it still has the old type of points, however.
This wire will run from the ‘points’ to the ignition amplifier that is strapped to the side of the HT coil in the photo. The ignition amplifier will then power the HT coil to create a spark.