Brake drum removal - Dually

[68_Step]

I am in the process of giving my 78 Chevy 1-Ton dually a rear brake job. I have it on jacks stands, the tires off and have removed the axle shafts from both sides. I think I have to remove the nut in the axle tube also, is that correct? If yes, how do I remove the "nut"? It is circular with round indentations in it - is it a sort of spanner wrench? I can also see a lock washer with a tab bent over holding it in place, that part looks easy

Thanks for the help!