Wire harnesses. When did the design change?

I sold a 65 Valiant convertible to a guy and will be helping him get it running. I got it in pieces so I am not all that familiar with how different it is from the 67 and later A bodies...of which I am much more familiar. The buyer is less experienced than me and is looking to me to steer the course.
I had a 65 Dart and didn't do much work on it to learn but I did notice that the bulkhead connector had only 2 plugs to it compared to 3 like the later A body cars had. The Dart also had 2 positive wires in between the bulkhead plugs...the wires attach to some type of steel studs that pass through the bulkhead. The bulkhead plugs look and fit different than the later A body cars. I have never owned a '64 or '66 A body so I am curious about the evolution of the design of these electrical systems.
When did this 1965 design begin and end? My '67 Dart seems to have the style the later cars had. I recall seeing 1976 A body cars that had a modified bulkhead to accommodate a 4th connector. At that point they must have exceeded the limits of the 24 pin design and didn't want to tool up for a change on a car that was in it's final year.