6401 (what I have) are non-aero, and were the first in the series, the 6403's are the aero ones and came a few years (?) after the 6401's, possibly to upgrade the Ultegra/600 group to match the look of the contemporary Dura Ace 7400.
@Dammit started
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6401 (what I have) are non-aero, and were the first in the series, the 6403's are the aero ones and came a few years (?) after the 6401's, possibly to upgrade the Ultegra/600 group to match the look of the contemporary Dura Ace 7400.