Hmm, after a bit or research, Slash!
As the Electra became operational, they found some excessive vibration in the centre cabin, and the outboard props were more highly stressed than the inboard ones - this was the reason the outboard nacelles' incidence was raised 3° upwards.
The Vno was reduced from 324 kt to 275 kt, and then a week later to 225 kt (with a Vne of 245 kt) after the second accident I mentioned above, and stayed that way until the LEAP was completed.