Cu/Ni keel cooler corrosion

[TugboatEng]

We are experiencing localized corrosion on our 90/10 copper nickel keel coolers. The coolers are isolated from the hull but tied to the hull and a 23lb zinc by a copper grounding strap from the ends. The erosion happens on the ends and is concentrated near the corners if the rectangular tube. The center of the cooler is also bolted to the hull, also isolated, with a 3/4-10 18-8 stainless nut (possibly 316 but vendors tend to go with whatever is available). The nut had pitting on the bottom surface and in the threads. Could this one nut cause this much galvanic corrosion in a period of about 8 years?

 

[TugboatEng]

This is what had to be removed.

 

[EdStainless]

In the first pic, is that braze filler or something else? It does not look like it was bonded to the tube. This would form a crevice without any galvanic protection.
What is the location of each part, how far away is the anode?

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P.E. Metallurgy, Plymouth Tube