Sorry, you will have to excuse my confusing statement as I am writing from. hospital bed and not at my sharpest!
To be clear, the original question related to maximum speed which I took to be Vd - 455 kts from about FL 60 up to about FL 360
This was the flutter clearance and was usually acheived in a dive. Vmo was thenwhat you got by backing off to give the statutory margins. Not strictly a flutter limit though limited by flutter! Vmo of course could be flown dry