CB uses the word "Spanner" which us limeys would reckon to be either an open ended or ring type. The access to that stud is easy with a modern day socket, but back in the day that was not what the average owner had to hand. This long nut enables the standard open ender in the standard toolkit to do the job.
But in the usual assembly process a standard nut is fine, as the dynamo can go on after the engine plates. The only reason you'd need access from time to time is to check tighten with a standard spanner and the dynamo in place. These days a thin wall socket does the job.
The term is also used as a tem of abuse in some circles, generally to draw attention to a person's failing in completing an important task. Example... "Why did you do that, you t*rucking spanner?
Swarfy.
*Typo.