The additional shrapnel protection actually dates back to early 1952 and was retrofitted to AD-3 and AD-4 Skyraiders as described in this May 1953 Naval Aviation News article.
This was the kit, as shown on the AD-6 Standard Aircraft Characteristics chart:
One way to tell if an AD has the deflector plate kit added is the step on the side of the fuselage. If it's a kick-in door, it hasn't been; if it's external, like a little ledge, then it has been.
An AD-4 that does not have the kit:
An AD-6 with the kit:
Crop from a Pete Bulban photograph
One AD configuration for the nuclear-strike mission was to 1) remove the deflector plate kit, the AERO pylons on the outboard wing panels, at least two of the 20mm cannon, and anything else not mission-critical that added weight and/or drag and 2) cover any holes with speed tape.
The AD-5 armor kit was slightly different because of the relocation of some engine accessories. See https://tailhooktopics.blogspot.com/2017/08/douglas-ad-5-armor.html