Update: I have been able to find an old Nissan part/tool number in my Chilton's. #KV40104400. Google has no relevant results, so it either never existed, or it has been redacted to cover up some sort of conspiracy theory as far as I can tell.
To clarify, it is the axle nut, aka hub nut, aka spindle nut for the rear axle. It is circular, not hex. It has 6 holes spaced evenly around it, two of them threaded to retain the lock washer. I'm aware of the hacks to bypass this problem and have used such methods successfully on beater rides in a pinch. However, I'm not treating this project as a beater ride and would rather not beat up my nearly pristine original parts. I've found several sockets that come close, but none just quite right to modify easily and properly. I am looking for info on sourcing the proper tool, or one that can be easily modified to perform properly. I will find the solution I'm looking for and will update. If I end up making the tool from scratch, I will. Thanks.