Actually, not having the diffrings keyed to the hub is a good thing, it lets the rings slip a little against the hub instead of the balls and so prolonging the time between rebuilds. If you have raced offroad you proberly know that the ae stealth diff is the best there is and it has this design, it only took losi 15+ years to figure out why
As for the belt stabilizer, it is what the name implies a stabilizer to prevent the belt from flipping over under high loads. It was first introduced on the 415ms as the original 415 had some issues with beltflipping.