I haven't ever understood what the hp had to do with the bolt falling out of the end of the axle.....why didn't it happen on 4020s?  It looks to me like it was simply not a good idea to leave the bolt loose and depend on the sheetmetal lock plate to keep it from turning.   Our 4430 did it before 2000 hours and I doubt it did much more work then what a lot of 4020s were doing.  It's never been a problem since it is put together with the bolt tight on a shim stack. 

Do you remember what the time frame was on the wider final drives?  I wonder if ours got them or not.   I know our 7520 is the same as a 4020, because it has one 4020 final on it where we broke an axle housing.