475 Indicated Air Speed, not TASSo that's a pretty low dive speed limit, given that Spitfires could be dived at over 500 mph without issue (and at least one was dived at over 600 mph without damage to plane or pilot). I wonder what gives there. It should also be noted at the 8g/8000 lbs limit the P-51D's g limit for combat weight drops to as low as 6.3 gs, and had no trouble with high speed dives.
BTW, I wish that such details were accessible and such for all the late Mustangs and even the Twin Mustang. Lots of tech drawings and info about earlier variants, not nearly as much as the variants after the D model.