ALL 56 Corvettes with the optional 2x4 carb engine had 9fin covers.
ALL 57 Corvettes with ANY optional engine got 9fin covers up to about mid production (around Dec 56) and then changed over to the 7fin covers.
Fairly early in the 57 FI cars with 9fin covers, the LEFT cover got a big notch machined in the upper fin to facilitate clearance of the FI air meter. It is generally believed that because of this notch in the left FI valve cover a decision was made to eliminate the upper/lower fins on all valve covers. Thus, the change from 9 to 7fins.