autolite

My project is a 1958 pick up. I want to service the generator but cant figure out this nut. Do I need a special tool?

Simple n00b question...I'm going through electrical troubleshooting steps on my 1948 Truck that hasn't ran in 10 years and have pulled my original autolite mz-4137 starter because it hasn't been out in 40 years or so and I would like to service and test it on the bench. My suspicion is that it's...