[ I put the meter between the negative battery post and the bonding wire. Got no voltage and no milliamps. Put the meter between the negative battery post and the quadrant got the same third of a volt and 33 milliamps going from the battery to the quadrant (?), and that is with all of the other negative leads taken off the post. I have a full keel boat with the rudder attached to the keel, so it has a lower gudgeon. Zinc today is two weeks old. The one previous to this one lasted only three weeks before I needed to replace it. I'm diving on it later today, but I'll bet it needs replacing again. Boat hasn't changed. Only thing different is that we got some new neighbors at the dock. I don't get why I would get a positive flow from the negative battery post to the quadrant. ]
If you put the meter between the battery post and the bonding system and you saw no reading then that means that they are at the "same potential". No current will flow unless you have a "difference in potential". Current is like water, it flows. Or, more likely, there is an open between your bonding system and the negative or ground side of your battery. Your bonding system is attached to your negative system (and to your zink) somewhere. Probably at the engine. But, your quadrant doesn't seem to be connected to your to your bonding system either.
I would expect a few milliamps between your bonding system and the battery because nothing is that clean. You always have a few connections in any circuit that could be made better so you always end us with some difference. Having "no difference" makes it a suspect reading. If you have an open between the bonding system and the negative system then you will have a difference in potential allowing voltage to flow.
I would connect everything back up. Put your meter between the bonding system and the block or your engine. You should have a minor reading of a few Milliamps indicating the two systems are connected. You could even test it with a buzzer. You should have no problem knowing it's connected. If you show them separate you have to find the open. Then I would connect a wire from the rudder shaft to the bonding system, maybe at the engine to start, and see if the reading goes away. Then try it at some other locations on the bonding system like various through hulls. in any event, the rudder shaft needs to be connected to the bonding system or be protected on it's own which doesn't seem to be working for you. I always ask the question "whats changed?" from the historical norm.
If you have a new boat in close proximity, try taking the boat out to anchor and running your tests again eliminating the stray currents associated with the dock AC and your neighbors. I would do that anyway just in case something changed on the dock AC. It happens all the time. Good luck.