https://imgur.com/RFmMomv
So I removed the two left most blade connectors to get the thick black wires out of the way. It appears that the second one from the left melted the plastic housing of the terminal. The insulation on the black wire going across the terminals also appears to be melted a bit.
Recalling my moment of brilliance, what I did specifically (99% sure) was arc the negative terminal of the starter battery across the 24v dump batteries when my wrench dropped out of my hands. It's been so long since I've thought about Kirchoff's rules and circuit theory that I'm not exactly sure what the implications of that are, simply from the stand point of where all the current went, but perhaps it is helpful to you smarter folk. I know the wrench itself took a fair amount of current as it arc welded itself to the dump battery terminal it touched.
The other observation I would note is that the sparks that came out of the controller box appeared to come out of the penetrations in the box where the dump battery cables go through. With that in mind, I have been wondering whether replacing the bank of four 35v, 1200 microfarad capacitors at the bottom of the box (Target70, is that what you are calling "cca?") would be the fix, but I expect it likely isn't nearly that simple.
I'm hoping by tomorrow, I can report on what happens when I connect the controller box up to a 12v battery. More to come.