Note: I've revised this entry in response to a lengthy comment that I have not posted because it contained minor errors, some of which were the result of my less than crystal clear statements in the original and verbally hand-waving the location of the exterior angle-of-attack indicator lights on the A4D-2N and subsequent. Since I'm not sure that I can delete a personal email address in a comment before posting it and some people may not want their email address made public, if someone wants a personal response to their question/comment, feel free to email me at email@example.com
I've been busy writing a book for Specialty Press on the Scooter. The A-4A/B corresponds to the A4D-1 and -2; The A-4C was previously the A4D-2N and the A-4E, the A4D-5. Note also the inlet difference on the J52-powered A-4E/F. More to come.
The A4D-2N (A-4C) nose was lengthened to accommodate a terrain avoidance radar and an access door added on the right side. It was originally produced with a large dark radome. The pitot that was located in front of the windscreen was moved to the left and a total temperature added to its right.
For some reason, probably to reduce weight or improve avionics access, the original A4D-2N nose was modified to replace the large dark radome with one shaped the same but possibly made of different material. The area defined by a paint or a erosion resistant coating was smaller, usually with a distinctive slant. The change roughly corresponded to the designation change to A-4C in 1962. However, large dark radomes are seen on deployed A-4s through at least 1965 that had not yet been through overhaul after the change was implemented.
(For an update on the radome story see http://tailspintopics.blogspot.com/2012/10/skyhawk-details.html)
There were other detail improvements like the addition of a windshield wiper. Another detail external difference between the A4D-2 and -2N was the angle of attack sensor and the external angle of attack indication for the LSO. On the A4D-2, the angle of attack transducer was a small "peg" (left picture below) in approximately the same location as the A4D-2N's "weather vane" and the light display was mounted on the nose gear strut (center picture below).
On the A4D-2N, it was a large weather vane as shown here. (Note also the replacement radome that has not yet been painted to match the forward fuselage.)
The three-light external display was relocated from the nosewheel strut to the left wing's inboard leading edge. In the following picture, it is the dark window on the wing leading edge just above and on the inboard side of the forward end of the main landing gear fairing.
For some background on the use of red over time to identify crush-point danger, see http://tailspintopics.blogspot.com/2012/10/painting-crush-points-red.html