The Belleville (spring) washers were in the original diff. They were replaced with a spring at about the JRXS type R time. Either one works fine. Put the pointy end of each washer toward the nut. Stack the six washers (not like your diagram). Tighten until it locks (flattens the washers), loosen 1/4 turn or so. My kit had the Belleville washers as well.

note the diff is easily adjusted without any dissasembly. Put a thin allen in the left side slot of the outdrive. Turn the right side wheel until the allen falls into the screw slot. Now tighten the diff by holding the allen and turning that right hand wheel. If your front diff is locked check it before every race day. It will loosen as the diff rings squeeze the gear plastic over time. On a ball diff check it every once in a while. Several times when new.