Well, I only know that a lot of the early E models had the rounded canopy. E-1 for instance too. But perhaps E-3's were modded with new canopies later on.
My immediate guess of the new canopy design was to improve visual conditions. The old (round) model seems to have a lot of bars running across the screen accompanied with smaller windows to peer through. The E-4 canopy may have been a result of pilots' complaints!
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