GM calls the big washer (GM 370251) a "retainer - ring - bolt". I still can't get my head around that. So maybe, when driving the bearing home, it stops at the retainer. It basically stops the bearing from being driven further. If that is the case, then it is acting more like a spacer. Maybe it acts to position the outside end of leather seal to be in position underneath the deflector (5.815)? Captures dripping oil and lets it weep out to the face of the axle flange. The width of the leather seal may require a different retainer.
Maybe with a modern seal, we don't need a "retainer - ring - bolt".
