One of the most notable differentiators between the A4D-1 and -2 is the rudder. On the -1, it had a smooth exterior surface. On the -2, longitudinal stiffening ribs were riveted to either side of a central panel. The change was made because of rudder flutter that had been experienced early on in A4D-1 flight test. Recently, someone inquired as to how the A4D-1 was released for service with a rudder flutter problem and by chance, someone else asked about a feature on the A4D-1 rudder that I hadn't paid attention to before. It turns out that the feature was the answer to the question.
In passing, I also noted another rudder detail change from the XA4D-1 to the production A4D-1. The X and early production Skyhawks had a rudder trim tab with an actuating rod on the left side of the rudder.
The original solution to the rudder flutter problem was the addition of a dash pot-type damper just above the center rudder hinge. Note that the trim tab is still part of the design.