Sometimes the behind the arm nut needs to be in the ring's hole before the ring is located on the spider tabs. Sometimes this nut is shorter then the 4 others. Sometimes this nut has a knurled outer edge to better grip the ring's recessed hole. Sometimes the bolt is also shorter. Sometimes the teeth of the granny ring block tool access to this bolt so tightening it before mounting the inner ring is a good idea.

I never saw that advantage of this design. (Yes, I know the claimed weight and/or stiffness changes. I just don't think they are enough to offset the hassle.) Andy.