Disregard-solution found.
The problem was that the nut (not bolt as I originally thought) was just stripped. It is a 14 mm hex nut that goes over a stud into the K plane. Got the stripped nut off and all is well. I am leaving the full original post up in case anyone runs into a similar problem because there wasn't much online or in the boating forums about this particular anode.

Original post
This is probably a stupid question with a simple solution. Nonetheless, I can't figure out the answer. I am (trying to) replace the big cylindrical anodes on my K planes. There seems to be a circular bolt-type thing holding the anode itself onto the K plane. It is recessed in the anode and it is not part of the anode. There is another bolt that goes into this bolt that holds on the anode wire, which was easy enough to remove, but I have no idea how to get this circular-headed anchoring fastener out. I have tried every possible 6- and 12-point socket, no joy. Tried to put the smaller bolt back in and see if the larger bolt is threaded opposite, nope. Tried turning the anode itself, it just spins somewhat freely around this fastener. Looked on the other side of the K plane for a nut, nothing there. I am at a loss. Any tips? Here are pictures of what I'm dealing with, and a picture of the new anode for reference to show that the bolt-thing is not part of the anode assembly.

That's a 10-32 nut. Probably 3/8 socket.

Clean the stuff out of the cavity.

https://www.mercruiserparts.com/bam/...32085/8504/380